Zeolite-confined nano-RuO2: A green, selective, and efficient catalyst for aerobic alcohol oxidation

Article abstract of DOI:10.1021/ja0282691

The development of green, selective, and efficient catalysts, which can aerobically oxidize a variety of alcohols to their corresponding aldehydes and ketones, is of both economic and environmental significance. We report here the synthesis of a novel aerobic oxidation catalyst, a zeolite-confined nanometersized RuO₂ (RuO₂-FAU), by a one-step hydrothermal method. Using the spatial constraints of the rigid zeolitic framework, we sucessfully incorporated RuO₂ nanoparticles (1.3 ± 0.2 nm) into the supercages of faujasite zeolite. Ru K-edge X-ray absorption fine structure results indicate that the RuO₂ nanoclusters anchored in the zeolite are structurally similar to highly hydrous RuO₂; that is, there is a two-dimensional structure of independent chains, in which RuO₆ octahedra are connected together by two shared oxygen atoms. In our preliminary catalytic studies, we find that the RuO₂ nanoclusters exhibit extraordinarily high activity and selectivity in the aerobic oxidation of alcohols under mild conditions, for example, air and ambient pressure. The physically trapped RuO₂ nanoclusters cannot diffuse out of the relatively narrow channels/pores of the zeolite during the catalytic process, making the catalyst both stable and reusable.

Full text of DOI:10.1021/ja0282691

Published on Web 01/30/2003
2
Zeolite-Confined Nano-RuO : A Green, Selective, and
Efficient Catalyst for Aerobic Alcohol Oxidation
†
,†
‡
†
Bi-Zeng Zhan, Mary Anne White,* Tsun-Kong Sham, James A. Pincock,
†
‡,§
†
Ren e´ J. Doucet, K. V. Ramana Rao, Katherine N. Robertson, and
T. Stanley Cameron†
Contribution from the Department of Chemistry and Institute for Research in Materials,
Dalhousie UniVersity, Halifax, NoVa Scotia, B3H 4J3, Canada, and Department of Chemistry,
The UniVersity of Western Ontario, London, Ontario, N6A 5B7, Canada
Received August 23, 2002 ; E-mail: Mary.Anne.White@dal.ca
Abstract: The development of green, selective, and efficient catalysts, which can aerobically oxidize a
variety of alcohols to their corresponding aldehydes and ketones, is of both economic and environmental
significance. We report here the synthesis of a novel aerobic oxidation catalyst, a zeolite-confined nanometer-
sized RuO
zeolitic framework, we sucessfully incorporated RuO
faujasite zeolite. Ru K-edge X-ray absorption fine structure results indicate that the RuO
anchored in the zeolite are structurally similar to highly hydrous RuO ; that is, there is a two-dimensional
structure of independent chains, in which RuO octahedra are connected together by two shared oxygen
atoms. In our preliminary catalytic studies, we find that the RuO nanoclusters exhibit extraordinarily high
activity and selectivity in the aerobic oxidation of alcohols under mild conditions, for example, air and ambient
pressure. The physically trapped RuO nanoclusters cannot diffuse out of the relatively narrow channels/
2
(RuO
2
-FAU), by a one-step hydrothermal method. Using the spatial constraints of the rigid
nanoparticles (1.3 ( 0.2 nm) into the supercages of
nanoclusters
2
2
2
6
2
2
pores of the zeolite during the catalytic process, making the catalyst both stable and reusable.
Introduction
Scheme 1. Aerobic Alcohol Oxidation
The aerobic oxidation of alcohols to their corresponding
aldehydes and ketones is of interest for both economic and
environmental reasons (Scheme 1). Many catalytic systems, for
II
1
2
3
example, M -radical catalysts, CuCl‚Phen, polyoxometalates,
4
5
6
PdLn, M-TEMPO, Ru-biomimetic-coupled systems, bimetallic
Mo-Cu and Os-Cu systems, perruthenate,9 Pt and Pt/
7
8
requirement and convenient separations - but their applications
are limited, either to activated alcohols or by the need for pure
oxygen. The search for suitable green aerobic catalysts, espe-
cially without sacrificial reducing agents, is still a significant
challenge.
1
0
11
Bi catalysts, manganese oxide octahedral molecular sieves,
and Ru-hydroxyapatite,12 have been designed and developed for
the aerobic oxidation of alcohols. Heterogeneous catalytic sys-
tems3
e,9a,10-12
display advantages - for example, no cocatalyst
Recently, RuO2 has been receiving considerable attention for
†
Dalhousie University.
The University of Western Ontario.
Present address: Department of Physics, University of Rajasthan, India.
1
3-15
‡
§
its extraordinarily high catalytic activity.
For example, it
can efficiently catalyze the oxidation of CO with O2 below room
(
1) (a) Chaudhuri, P.; Hess, M.; M u¨ ller, J.; Hildenbrand, K.; Bill, E.;
Weyherm u¨ ller, T.; Wieghardt, K. J. Am. Chem. Soc. 1999, 121, 9599. (b)
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15
temperature. The outstanding catalytic ability of RuO2(110)
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(
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(
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2
001, 789. (b) Haimov, A.; Neumann, R. Chem. Commun. 2002, 876. (c)
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7
66. (b) Peterson, K. P.; Larock, R. C. J. Org. Chem. 1998, 63, 3185. (c)
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Soc. 2001, 123, 7188. (d) ten Brink, G.-J.; Arends, I. W. C. E.; Sheldon,
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W. C. E.; Sheldon, R. A. Science 2000, 287, 1636.
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S.; Thomas, A. W. Chem. Commun. 1999, 1907. (b) Lenz, R.; Ley, S. V.
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10.1021/ja0282691 CCC: $25.00 © 2003 American Chemical Society
J. AM. CHEM. SOC. 2003, 125, 2195-2199
9
2195

A R T I C L E S
Zhan et al.
is attributed to the existence of coordinatively unsaturated
ruthenium atoms.13 Interestingly, a structure similar to that of
1
6
RuO2(110) was found in amorphous hydrous RuO2. In
addition, RuO2‚xH2O is a mixed electron-proton conductor. It
can activate O2 molecules without the assistance of any
additional reducing agents.
Nanometer-sized materials can exhibit very different elec-
tronic, magnetic, catalytic, optical, and other properties from
17
those of the corresponding bulk materials, and nanomaterials
can be novel catalysts because of the large surface area and the
high density of active sites. Thus, it is expected that nanometer-
sized hydrous RuO2 clusters with a structure similar to that of
the RuO2(110) surface could exhibit favorable aerobic oxidation
chemistry. Here, we report a novel approach to directly
synthesize RuO2 nanoclusters in the supercages of FAU zeolite.
Catalytic results indicate that zeolite-confined nano-RuO2 is a
green, selective, and efficient aerobic catalyst for alcohol
oxidation. It can selectively oxidize alcohols to their corre-
sponding carbonyls under mild aerobic conditions without using
sacrificial reducing agents, pure O2, or even solvent.
Figure 1. XRD patterns of unmodified FAU zeolite and RuO2-FAU
composite.
mmol of alcohol, 3 mL of toluene, 0.1 g of Ru-FAU catalyst, and a
stir bar. The aerobic oxidation reactions were conducted in an 80 °C
oil bath, in air (except as indicated) under ambient pressure. The
oxidation products were analyzed and quantified by gas chromatography
and identified by either GC-MS or GC with standard samples. All of
the GC analyses were performed on a Supelco MDN-55 column (30
m × 0.25 mm × 0.50 µm) with a Perkin-Elmer Auto System GC
equipped with an FID-detector. GC-MS for the product identification
was conducted in a Perkin-Elmer Auto System XL GC with a Perkin-
Elmer TurboMass mass spectrometer.
Experimental Section
Chemical reagents included fumed silica (11 nm, Sigma), tetraethyl
orthosilicate (Aldrich), NaOH (Aldrich), NaAlO
Al(OH) (McArthur Chemical), RuCl ‚3H O (Aldrich), Ru(NH
Aldrich), and RuO (A. D. Macky). All other organic and inorganic
chemicals were reagent grade and were used without further purification.
Synthesis of RuO -FAU. RuO nanoclusters confined in the
supercages of FAU zeolite, hereafter called RuO -FAU composite,
were synthesized on the basis of the organic-additive-free hydrothermal
2
(Allied Chemical),
3
3
2
3
)
6
Cl
3
(
2
2
2
Characterization. X-ray powder diffraction patterns were recorded
on a Rigaku Miniflex System using Cu KR radiation, 30 kV, 15 mA
2
-
1
18
with a scanning speed of 1° (2θ) min , T ) 20 °C. Ru K-edge XAFS
crystallization method reported recently. A measured amount of either
RuCl ‚3H O or Ru(NH Cl was added to an aluminosilicate gel
containing 5.34 g of NaOH, 2.42 g of NaAlO , 3.43 g of SiO , and
0.0 g of H O. The gel was aged for 2 days and then crystallized at 90
C for 15 h with stirring. The resultant black powder was separated
measurements were made at the Bending Magnet Beamline of the PNC
3
2
3
)
6
3
(Pacific Northwest Consortium)-CAT (Collaborative Access Team) at
2
2
the Advanced Photon Source (APS) at Argonne National Laboratory.
APS is a 7 GeV, third generation electron storage ring, operating
typically at 100 mA injection current. All of the measurements were
conducted in fluorescence mode using a Xe-filled ion chamber with
filter and solar slit arrangements. High-resolution TEM images were
recorded with a FEI Tecnai-12 operated at 80 kV. The EPR experiment
was conducted with a Bruker ESP 300 Xband spectrometer.
5
2
°
from solution by centrifugation and then washed completely with
deionized (DI) water to remove any physically absorbed species on
the zeolite surface. The synthesized samples were dried at room
temperature for further characterization and catalytic investigations. ICP-
MS analysis indicates that the concentration of RuO
the zeolite is 0.78 mmol/g. This corresponds to about one RuO
nanocluster in every 2.2 supercages of FAU (on average, every 1.3
nm RuO nanocluster contains 5 Ru atoms, as deduced from the bond
lengths).
2
incorporated in
2
Results and Discussion
2
X-ray powder diffraction indicates that unmodified FAU
zeolite and the RuO -FAU composite have the same structure
2
Aerobic Oxidation. Unless otherwise indicated, the oxidation
reactions were carried out in a flask (with a condenser) containing 1
(Figure 1). In comparison with the unmodified FAU zeolite,
RuO2-FAU displays slightly higher 2θ values in all of its
diffraction peaks. Both diffraction patterns match very well with
(
10) (a) Keresszegi, C.; B u¨ rgi, T.; Mallat, T.; Baiker, A. J. Catal. 2002, 211,
1
9
2
2
1
44. (b) Jenzer, G.; Sueur, D.; Mallat, T.; Baiker, A. Chem. Commun. 2000,
247. (c) Mallat, T.; Bodnar, Z.; Hug, P.; Baiker, A. J. Catal. 1995, 153,
31. (d) Kluytmans, J. H. J.; Markusse, A. P.; Kuster, B. F. M.; Marin, G.
that simulated for faujasite zeolites. X-ray fluorescence
analysis shows that the Si/Al ratio is 1.25 for FAU and 1.34
for RuO2-FAU, within the range of 1.0-1.5 for faujasite-X
zeolite. There is no evidence for any crystal phases attributable
to RuO2 compounds, indicating that zeolite-confined RuO2 is
not highly crystalline.
Ru K-edge X-ray absorption fine structure (XAFS) was
employed to study the structural configuration of the Ru species
incorporated in the FAU zeolite. Figure 2 shows the X-ray
absorption near-edge structure (XANES) spectra of various Ru
species. The XANES spectra of synthetic RuO2-FAU com-
posite materials are very different from those of RuCl3 and
Ru(NH3)6Cl3 but resemble that of hydrous RuO2. There is no
B.; Schouten, J. C. Catal. Today 2000, 57, 143. (e) Besson, M.; Gallezot,
P. Catal. Today 2000, 57, 127. (f) Lee, A. F.; Gee, J. J.; Theyers, H. J.
Green Chem. 2000, 2, 279.
(
(
(
(
11) Son, Y.-C.; Makwana, V. D.; Howell, A. R.; Suib, S. L. Angew. Chem.,
Int. Ed. 2001, 40, 4280.
12) Yamaguchi, K.; Mori, K.; Mizugaki, T.; Ebitani, K.; Kaneda, K. J. Am.
Chem. Soc. 2000, 122, 7144.
13) Over, H.; Kim, Y. D.; Seitsonen, A. P.; Wendt, S.; Lundgren, E.; Schmid,
M.; Varga, P.; Morgante, A.; Ertl, G. Science 2000, 287, 1474.
14) Madhavaram, H.; Idriss, H.; Wendt, S.; Kim, Y. D.; Knapp, M.; Over, H.;
Aâmann, J.; L o¨ ffler, E.; Muhler, M. J. Catal. 2001, 202, 296.
15) Zang, L.; Kisch, H. Angew. Chem., Int. Ed. 2000, 39, 3921.
16) Mckeown, D. A.; Hagans, P. L.; Carette, L. P. L.; Russell, A. E.; Swider,
K. E.; Rolison, D. R. J. Phys. Chem. B 1999, 103, 4825.
17) Goldstein, A. N. Handbook of Nanophase Materials; Marcel Dekker: New
York, 1997.
(
(
(
(
18) (a) Zhan, B.-Z.; White, M. A.; Robertson, K. N.; Cameron, T. S.;
Gharghouri, M. Chem. Commun. 2001, 1176. (b) Zhan, B.-Z.; White, M.
A.; Lumsden, M.; Mueller-Neuhaus, J.; Robertson, K. N.; Cameron, T. S.;
Gharghouri, M. Chem. Mater. 2002, 14, 3636.
(19) Treacy, M. M. J.; Higgins, J. B.; von Ballmoos, R. Collection of Simulated
XRD Powder Patterns for Zeolites, 3rd ed.; Elsevier: New York, 1996.
2196 J. AM. CHEM. SOC.
9
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Zeolite-Confined Nano-RuO
2
A R T I C L E S
Scheme 2. Ru, Larger Six-Coordinate Spheres; O, Smaller
Spheres; Si, Two Spheres Indicated for RuO2-FAU
RuO2‚2.32H2O, which has been systematically studied by
1
6
McKeown et al. It has a two-dimensional structure of
independent chains, in which RuO6 octahedra are connected
together by two shared oxygen atoms (Scheme 2). The EXAFS
results indicate that the Ru(IV) species anchored in the zeolite
likely form amorphous RuO2 with a 2D-chain structure,
consistent with the XRD findings. The main peak in the FT
corresponds to a Ru-O interatomic distance of 1.91 ( 0.01 Å.
Analysis using phase shift from the standard shows that the
Ru-O bond in RuO2-FAU is slightly shorter on average (by
Figure 2. X-ray absorption near-edge structure (XANES) spectra for
different types of Ru species, as indicated.
0
.02 Å) than that of hydrous RuO2. The shorter Ru-O bond of
RuO2-FAU therefore leads to a shorter Ru-Ru distance of 3.05
0.01 Å, as compared with 3.15 Å for hydrous RuO2. The 2.2
Å peak, only present in highly hydrated RuO2, was assigned to
(
1
6
the Ru-OH2 or the Ru-OH bond. The corresponding bond
length is 2.57 Å in the RuO2-FAU composite, longer than 2.51
Å in the hydrous RuO2 reference. Reasonable explanations for
the longer Ru-OH2 or Ru-OH bond and shorter Ru-O and
Ru-Ru bonds in RuO2-FAU are the very polarized zeolitic
Figure 3. The radial distribution function from the Fourier transform of
2
0
2
framework and the strong interaction between RuO2 chains
the k -weighted extended X-ray absorption fine structure (EXAFS) for
RuO2-FAU and hydrous RuO2.
and the zeolitic framework in the form “Ru-O-Sizeo”, where
21
Sizeo is a zeolitic framework atom. The interactions could occur
either on the terminal oxygen atoms or on the bridge oxygen
atoms as shown in Scheme 2. This interaction may be the driving
spectral difference between the two RuO2-FAU samples,
synthesized with either RuCl3 or Ru(NH3)6Cl3, indicating that
all ruthenium species are oxidized to Ru(IV) under the hydro-
thermal crystallization conditions. The absence of Ru(III) species
in the RuO2-FAU composites was also evidenced by the EPR
experiment: all of the synthesized composites are EPR silent
at both room temperature and 120 K. The similarity of the
XANES spectra of the RuO2-FAU composites and hydrous
RuO2 clearly demonstrates that hydrous RuO2 and zeolite-
confined Ru species have similar nearest-neighbor octahedral
environments, that is, distorted “RuO6”.16
force for the successful inclusion of RuO nanoclusters into the
2
zeolitic supercages during the hydrothermal synthesis.
The morphology and particle sizes of RuO clusters in FAU
2
zeolite were characterized using high-resolution TEM, as shown
in Figure 4. The additional dark spots in RuO -FAU crystals,
2
not seen in pure FAU but homogeneously distributed through
the RuO -FAU sample, are reasonably attributed to the RuO2
2
particles. Figure 4 clearly indicates that these particles are very
uniform, in the size range 1.3 ( 0.2 nm. This is the same size
Extended X-ray absorption fine structure (EXAFS) was
further used to define the fine structure of RuO2 anchored in
the zeolite. Figure 3 shows the radial distribution function (RDF)
from the Fourier transform of the k -weighted EXAFS for
RuO2-FAU, along with the hydrous RuO2 standard. The
as the supercages in FAU zeolite. The RuO clusters are likely
incorporated into the supercages of FAU zeolite during the
hydrothermal crystallization process, and the growth of RuO2
2
2
(
20) Uppili, S.; Thomas, K. J.; Crompton, E. M.; Ramamurthy, V. Langmuir
000, 16, 265.
(21) Dutta, P. K.; Das, S. K. J. Am. Chem. Soc. 1997, 119, 4311.
2
EXAFS of the latter is virtually the same as that for amorphous
J. AM. CHEM. SOC.
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A R T I C L E S
Zhan et al.
Table 1. Aerobic Oxidation Results of Alcohols Using RuO2-FAU
Catalyst
Figure 4. High-resolution TEM image of RuO2-FAU composite.
clusters is constrained by the rigid zeolitic framework. Further-
more, due to the structure of FAU zeolite, with larger supercages
tetrahedrally connected by smaller 12-membered rings, once
incorporated, the nanoclusters of RuO2 are physically confined
in the supercages and cannot diffuse out through the relatively
narrow channels. This physical occlusion, supported by the
catalytic results (vide infra), is of significance in the prevention
of leaching of the active sites, a common problem for supported
transition-metal catalysts.22
Various activated (benzylic and allylic) and unactivated (satur-
ated) alcohols were used to investigate the aerobic oxidation
over RuO2-FAU. Table 1 presents the aerobic catalytic results.
In a control experiment, unmodified faujasite zeolite was used
as a “catalyst” for aerobic oxidation of benzyl alcohol. No oxi-
dation product was detected from GC analysis after 4 h of reflux-
ing, indicating that the unmodified zeolite itself is inert for
alcohol oxidation under the experimental conditions (entry 1).
Using pure hydrous RuO2, that is, not encapsulated in the zeolite,
as a catalyst, we found that 16% of the benzyl alcohol was
oxidized to benzaldehyde within 1.5 h at 80 °C.2 However,
benzyl alcohol was completely oxidized to benzaldehyde within
a
Conversion of alcohols. b Turn over number based on the substrate turn
c
over per mole of RuO2. Unmodified zeolite (0.1 g) was used as “catalyst”.
d
e
O2 bubbling. Alcohol (30 mmol), RuO2-FAU (0.3 g), 80 °C, air
f
atmosphere. Alcohol (1 mmol), RuO2-FAU (0.1 g), toluene (10 mL),
g
ambient temperature, O2 bubbling. Chlorobenzene (10 mL) instead of
3
h
toluene solvent. 4-Picoline (3 mmol) added to the reaction mixture.
Oxidation result for the fifth run.
i
1
.5 h with more than 99% selectivity when RuO2-FAU was
used as catalyst under the same reaction conditions (entry 2).
Dried RuO2-FAU gave the same results as entry 2.) These
ldehyde (entry 4). In comparison with unactivated primary al-
cohols, we found that secondary noncyclic aliphatic alcohols
are more reactive to oxidation by RuO2-FAU: 69% of 2-hep-
tanol was selectively oxidized (to 2-heptanone) in 4 h (entry
(
results clearly indicate that zeolite-confined nano-RuO2 is much
more active than the bulk RuO2 and that the Ru species incor-
porated in the zeolitic frameworks are the catalytically active
sites for the alcohol oxidation. The significantly higher activity
of nano-RuO2-FAU composite, as compared with that of bulk
hydrous RuO2, is attributed to the much higher density of active
sites in nano-RuO2.
We note that RuO2-FAU also displays high activity in the
oxidation of unactivated alcohols (Table 1, entries 3-8). For
example, n-heptanol was selectively oxidized to n-heptaldehyde
with 44% conversion in 4 h (entry 3). After 20 h, the reaction
was essentially complete with over 99% selectivity for n-hepta-
5
). This trend is different from that observed in the monomeric
5
d
12
ruthenium catalysts, such as RuCl2(PPh3)3 and RuHAP.
However, only 17% of cyclohexanol was selectively oxidized
to cyclohexanone in the same reaction period (entry 6). This is
2
4
further confirmed by a competitive reaction, which indicates
that 1-heptanol is about 3 times more active than cyclohexanol
with the RuO2-FAU catalyst. This result matches very well
with the individual reactions (entries 3 and 6). The oxidation
reactions can be significantly promoted in a pure O2 atmo-
sphere: the conversions were 43% for cyclohexanol and 98%
for n-heptanol in 4 h when the oxidation was conducted with
an O2 atmosphere (entries 7 and 8), that is, about 3 times faster
than that conducted in air (entries 3 and 6).
(
22) (a) Schuchardt, U.; Cardoso, D.; Sercheli, R.; Pereira, R.; da Cruz, R. S.;
Guerreiro, M. C.; Mandelli, D.; Spinac e´ , E. V.; Pires, E. L. Appl. Catal.,
A: General 2001, 211, 1 and references therein. (b) Thomas, J. M.; Raja,
R. Chem. Commun. 2001, 675 and references therein.
(
23) The control reaction was conducted in a 25 mL flask containing 0.078
(24) The competitive reaction was done by mixing together 0.5 mmol of
mmol of hydrous RuO
2
, 1 mmol of benzyl alcohol, and 3 mL of toluene
n-heptanol and cyclohexanol with 3 mL of toluene solvent and 0.1 g of
solvent. The mixture was heated in air at 80 °C with stirring. GC analysis
indicated that 16% of benzyl alcohol was selectively oxidized to benzal-
dehyde within 1.5 h.
2
RuO -FAU catalyst. The reaction was carried out at 80 °C in air with
stirring. GC analysis indicated that the conversions of n-heptanol and
cyclohexanol in 2 h were 32 and 12%, respectively.
2198 J. AM. CHEM. SOC.
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Zeolite-Confined Nano-RuO
2
A R T I C L E S
The corresponding allylic alcohol is oxidized much more
easily: 68% of 2-cyclohexenol was selectively oxidized to
The catalytic results indicate that there is no difference in either
activity or selectivity between the first and fifth runs (entries 2
and 16). Ru leaching was negligible during the above recycles.
The Ru concentration in the filtrate is less than 2.5 ppb as found
by ICP. X-ray powder diffraction patterns indicated that the
crystallinity of recycled RuO2-FAU sample is the same as that
of the fresh sample, showing that the zeolitic framework is
retained in the oxidation process. Although the mechanism of
alcohol oxidation over RuO2-FAU catalyst is not very clear,
we think that it is similar to that of bulk RuO2 as proposed by
2-cyclohexenone in 4 h over RuO2-FAU (Table 1, entry 9),
about 4 times the reactivity of cyclohexanol. The aerobic
oxidation of 2-cyclohexenol was completed in 8 h (entry 10).
The selective oxidation of 2-buten-1-ol to 2-butenal occurred
with 95% conversion in 4 h (entry 11). Furthermore, aerobic
oxidation can be carried out without using an organic solvent.
For example, neat benzyl alcohol was selectively oxidized to
benzaldehyde with a conversion of 22% in 20 h. The TON now
reaches 28 (entry 12).
1
4
Madhavaram. Aerobic oxidation is first initiated by OH
dissociation leading to the formation of a Ru-alcoholate species.
Our evidence for this is inhibition of benzyl alcohol oxidation
in the presence of 4-picoline and no reaction for the oxidation
of 9-hydroxyfluorene due to the steric effect. The alcoholated
species, then, undergoes dehydrogenation to produce the cor-
responding carbonyl compound and ruthenium hydride. The
latter can either react with Ru-OH species to give water and
The RuO2-FAU catalyst is so active that it can catalyze the
aerobic oxidation of benzyl alcohol at ambient temperature
(Table 1, entries 13-15). The substrate is selectively oxidized
to benzaldehyde with 58% conversion in 24 h (entry 13). This
oxidation reaction is noticeably enhanced using chlorobenzene
as solvent: a conversion of 75% was achieved under the same
conditions (entry 14). On the other hand, addition of tertiary
amines, which coordinate more strongly to Ru than alcohols,
can inhibit the reaction. The oxidation reactivity is significantly
dropped to 23% by adding 3 mmol of 4-picoline to the reaction
mixture (entry 15).
1
4
then uptake oxygen to finish a catalytic cycle or react with
1
2
O2 through a hydroperoxide route.
Conclusion
The RuO2-FAU catalyst also displays substrate shape-
selectivity imposed by the zeolitic framework. In a competitive
reaction, equimolar (0.5 mmol) benzyl alcohol and 9-hydroxy-
fluorene were mixed with the RuO2-FAU catalyst (0.1 g) in
toluene (10 mL) at 80 °C, air atmosphere. GC analysis indicated
that the benzyl alcohol was completely and selectively oxidized
to benzaldehyde in 1.5 h, and no oxidation products attributed
to 9-hydroxyfluorene were detected. However, 9-hydroxyfluo-
rene was readily oxidized to the corresponding ketone when
pure hydrous RuO2 was used as catalyst.25 The 9-hydroxyfluo-
rene molecule is rigid and bulky and can only pass through the
narrow 12-member ring zeolite channel in one orientation,
leaving the hydroxyl group too far away from the active site of
the RuO2 cluster to be oxidized. This shape-selectivity on the
substrates of the RuO2-FAU catalysis provides strong evidence
that the RuO2 nanoclusters are confined in the supercages of
the FAU zeolite and that any physically adsorbed RuO2 species
on the zeolite surface are removed by washing with DI water.
Furthermore, the RuO2-FAU catalyst is stable and can be
reused after aerobic oxidation. After reaction with benzyl
alcohol, RuO2-FAU was separated from the reaction mixture
by centrifugation, thoroughly washed with acetone, and then
reused as catalyst for the next run under the same conditions.
Nanometer-sized hydrous RuO2 was successfully incorporated
into supercages of faujasite zeolite using a one-step hydrothermal
synthesis. Zeolite-anchored hydrous nano-RuO2 has a 2D-chain
structure, in which the RuO6 units are connected together by
two shared oxygen atoms. The zeolite-confined nano-RuO2
displays extraordinarily high activity and selectivity in the
oxidation of both activated and unactivated alcohols to the
corresponding aldehydes and ketones under aerobic conditions
without using either a cocatalyst or a sacrificial reducing agent.
Zeolite-confined nano-RuO2 is much more active than bulk
hydrous RuO2 in the aerobic oxidation of benzyl alcohol under
the same conditions. The Ru leaching was observed to be
negligible, and the crystallinity of recycled RuO2-FAU was
retained, showing that the RuO2-FAU catalyst can be reused.
Acknowledgment. The authors thank the U.S. Department
of Energy and NSERC (MFA grant) and Dr. Robert Gordon at
PNC-CAT for XAFS support and assistance, and Dr. R.
Sammynaiken, Saskatchewan Structure Sciences Center, Uni-
versity of Saskatchewan for EPR. K.V.R.R. acknowledges the
Center for Chemical Physics at the University of Western
Ontario for a Visiting Fellowship. This work was also financially
supported by the NSERC and the Killam Trusts (the latter for
a postdoctoral research fellowship to B.-Z.Z. and a research
professorship to M.A.W.).
(
25) The reaction was done by mixing together 0.078 mmol of hydrous RuO
.5 mmol of 9-hydroxyfluorene, and 10 mL of toluene solvent. The mixture
was heated in air at 80 °C with stirring. GC analysis indicated that 51% of
-hydroxyfluorene was selectively oxidized to 9-fluorenone within 1.5 h.
2
,
0
9
JA0282691
J. AM. CHEM. SOC.
9
VOL. 125, NO. 8, 2003 2199