Shape-selective oxidation of primary alcohols using perruthenate-containing zeolites

Article abstract of DOI:10.1039/b212866g

Potassium perruthenate (KRuO₄), a known, effective oxidant for the conversion of primary and secondary alcohols into carbonyl compounds is impregnated into zeolite X and shown to be a shape-selective oxidant using benzyl alcohol (reacted) and pyrenemethanol (not reacted).

Full text of DOI:10.1039/b212866g

Shape-selective oxidation of primary alcohols using
perruthenate-containing zeolites
Daniel L. Wu, Andrea P. Wight and Mark E. Davis*
Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA
Received (in Purdue, IN, USA) 27th December 2002, Accepted 5th February 2003
First published as an Advance Article on the web 25th February 2003
Potassium perruthenate (KRuO4), a known, effective oxi-
dant for the conversion of primary and secondary alcohols
into carbonyl compounds is impregnated into zeolite X and
shown to be a shape-selective oxidant using benzyl alcohol
(reacted) and pyrenemethanol (not reacted).
varied from 2 Ru atoms/supercage (Ru-FAU-2) to 10 Ru atoms/
supercage (Ru-FAU-10). The impregnated zeolites were char-
acterized by UV-visible spectra (Cary 3G UV-visible Spec-
trophotometer), nitrogen adsorption isotherms (collected at 77
K on an Omnisorp 100 analyzer), thermogravimetric analysis
(Netzsch STA 449C) and XPS (performed by the Chevron
Research and Technology Co.).
Oxidation of alcohols has been the focus of much research,
especially the search for suitable catalysts that use more
environmentally friendly oxidants than the typical organic
peroxides or amine oxides.1 Catalytic systems, both homoge-
neous and heterogeneous, based on copper,2 ruthenium,3
palladium,4 rhodium,5 cobalt,6 and manganese,7 have been
reported to oxidize various alcohols into the corresponding
carbonyl compound using molecular oxygen as the oxidant. Ley
and co-workers showed that tetra-n-propylammonium perruthe-
nate (TPAP) can be a suitable homogeneous catalyst for the
room temperature oxidation of primary and secondary alcohols
with N-methyl-morpholine-N-oxide (NMO) as a co-oxidant.8,9
Additionally, homogeneous TPAP can use molecular oxygen as
a co-oxidant for alcohol oxidation.10 Heterogeneous oxidation
catalysts have also been prepared by supporting the perruthe-
nate anion on polymeric resins11,12 and inorganic supports13 via
anion exchange with potassium perruthenate (KRuO4). The
heterogeneous catalysts oxidize alcohols, using either NMO11
or molecular oxygen13 as a co-oxidant. Other methods of
immobilizing and recycling perruthenate catalysts have been
reported also, e.g., in ionic liquids14 or sol–gel materials,15
using either NMO14 or molecular oxygen14,15 as co-oxidants.
Here, we describe the first use of perruthenate in a zeolite
support, and show its use as a recyclable, shape-selective
oxidant in the room temperature oxidation of primary alcohols
to aldehydes. Zeolites are known for their uniform pore size and
ability to perform shape-selective catalysis. They also have
superior chemical and thermal stability compared to polymeric
supports. By confining the perruthenate anion within the zeolite
void space, it should be possible to create a solid that is stable
to regeneration and also able to discriminate between reactant
substrates on the basis of molecular size. Additionally, these
oxidizing solids will likely be compatible with other types of
catalysts such as enzymes and organometallic complexes to
allow the combinations of catalyst systems for performing
multiple reactions within the same vessel. This is because the
zeolite will isolate the oxidation function from contact with the
other catalysts that would themselves be oxidized by perru-
thenate.
The impregnated solids were tested as oxidants for the
conversion of benzyl alcohol 1 to benzaldehyde 2 and
1-pyrenemethanol 3 (too large to enter pores of zeolite) to
1-pyrenecarboxaldehyde 4 (Scheme 1). The reactions were
performed at room temperature under argon atmosphere in
dichloromethane (DCM) or dichloroethane (DCE) as solvent.
The reaction yield was monitored by GC (HP 5890 Series II,
with a HP-5 capillary column, 30 m 3 0.25 mm 3 0.25 mm),
using biphenyl as an internal standard (benzyl alcohol,
1-pyrenemethanol, biphenyl, DCM, and anhydrous DCE were
obtained from Aldrich). DCM was dried and de-gassed by
refluxing with calcium hydride and distillation under Ar. DCE
and benzyl alcohol were dried over activated molecular sieves
(4 Å, Advanced Specialty Gas Equipment) and then de-gassed
by 3 freeze–pump–thaw cycles under dry nitrogen.
Results from reacting benzyl alcohol are summarized in
Table 1 (aldehyde is the only product observed). A heteroge-
neous slurry of KRuO4 in DCM oxidizes the alcohol to
benzaldehyde giving a yield of 95% at 24 hours. Ru-FAU-2
gave a 3% yield of 2 in the first 24 hours of reaction and
increased to 8% after 76 hours of reaction. With a higher
loading of Ru, Ru-FAU-10 gave yields of 10 and 19% over 24
and 48 hour periods, respectively. K-X shows no activity for the
oxidation. Benzyl alcohol can readily access the pore space of
zeolite-X and react with the KRuO4 impregnated within the
void space. However, at room temperature, the reaction rate
Scheme 1
Table 1 Oxidation of benzyl alcohol 1 over Ru-containing materials (1
mmol 1: 1.5 mmol Ru, in 25 mL DCM)
Na-X (FAU) zeolite was purchased from Strem Chemicals
(Na88Si104Al88O384·220H2O, 7.4 Å pore diameter, 1.18 Si/Al).
The Na-X was ion-exchanged to the potassium form (K-X) with
0.1 M potassium hydroxide (KOH) in four batch exchange steps
(the final exchange was for 4 days). The K-X (1.261 g) was
dehydrated at 200 °C for 2 h under vacuum to remove adsorbed
water prior to the impregnation of perruthenate. A solution of
KRuO4 (Alfa Aesar) in methanol or acetone as solvent (0.259 g
KRuO4, 1.3 mmol, 50 mL solvent) was then added to the dried
K-X and allowed to stir for several hours to permit the
perruthenate to diffuse into the pore space. The solvent was then
evaporated under flowing argon overnight to yield Ru-
impregnated K-X (Ru-FAU). Loadings of the perruthenate were
Material
Reaction time/h
Yield of 2 (%)a
KRuO4
Ru-FAU-2
24
24
76
24
48
24
27
24
95
3
8
10
19
< 0.02
68
0
Ru-FAU-10
K-X
CPG-P+RuO4
CPG-240
2
a Relative to moles of 1.
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CHEM. COMMUN., 2003, 758–759
This journal is © The Royal Society of Chemistry 2003

Table 3 Regeneration and recycling of Ru-FAU-2 (1 mmol 1+1.5 mmol Ru,
could be limited by slow diffusion of benzyl alcohol within the
microporous void spaces. To test for this effect, a mesoporous
solid was prepared by ion-exchanging KRuO4 onto controlled
pore glass (CPG-240, 240 Å pore diameter) that was function-
alized with quaternary phosphonium chloride moieties (similar
to work by Bleloch et al. in the ammonium-functionalized
in 5 mL DCE)
Reaction Yield of
Material
time/h
2 (%)a
24
76
24
3
8
1
MCM-41 system).13 The RuO4 exchanged onto 47% of the
2
As-made Ru-FAU-2
phosphonium groups to give CPG-P+RuO42 that contained 0.27
mmol RuO42/g solid. The mesoporous material should have
minimal diffusion limitations and produced 68% yield of 2 in 27
hours of reaction (CPG-240 was not active for the oxidation of
benzyl alcohol). Thus, some effects of diffusion in the zeolite
catalysts are likely.
Spent Ru-FAU-2 (without regeneration)
Spent Ru-FAU-2 regenerated 35 mL min21 flowing
O2, 100 °C, 3 h
24
3
a Relative to moles of 1.
The shape-selective nature of the Ru-FAU solid was
demonstrated by testing for the reaction of pyrenemethanol 3
(Table 2). A DCE slurry of KRuO4 gave a 100% yield of 4 in 72
hours. Ru-FAU-2 shows no activity towards oxidation of 3 even
after 120 hours of contact. Ru-FAU-10 shows slight activity
(2% after 45 hours), but this is likely due to some amount of
KRuO4 on the surface of the zeolite (see below). The
mesoporous CPG-P+RuO42, that has sufficient pore size for
adsorption of pyrenemethanol, gave 100% yield of 4 in 24
hours. Like with benzyl alcohol, neither K-X or CPG-240
showed any activity. Thus, Ru-FAU is able to oxidize alcohols
into aldehydes in a shape-selective manner.
operation involving regeneration of the active site, RuO42, on
the spent solid could be done. In addition, it would be desirable
to use an economical oxidant, such as molecular oxygen, as the
co-oxidate or to perform the regeneration. Here, the Ru-FAU-2
solid was tested for regeneration in oxygen. The results are
illustrated in Table 3. As described above, as-made Ru-FAU-2
showed 3% yield of 2 in 24 h of reaction. After isolating the
spent solid from the reaction, no absorbance was observed at
300–330 nm in the UV-visible spectrum. The spent solid
showed little activity (1% yield) in a subsequent reaction with 1.
The spent solid was then regenerated at elevated temperature
with flowing oxygen for 3 h. After the spent solid was
regenerated at 100 °C for 3 h, the activity of the recycled
material was at levels observed from the fresh solid (3% yield
after 24 h) and the UV-visible spectrum revealed the absorbance
in the 300–330 nm range. Thus, the spent Ru-FAU-2 solid can
be regenerated at 100 °C in oxygen and recycled to oxidize
benzyl alcohol. While the reported yields are low, variations in
temperature and solid amounts will certainly provide for higher
conversions in shorter reaction times. Here, we provide results
leading to the proof of concept and not optimal performance.
D. L. W. and A. P. W. acknowledge support from a California
Institute of Technology Summer Undergraduate Research
Fellowship and a National Science Foundation Graduate
Fellowship, respectively.
KRuO4 (neat) shows a broad absorbance in the UV-visible
spectrum from 300–330 nm. K-X, CPG-240, and RuO2 (the
fully reduced form of RuO42) have no absorbances over the
UV-visible spectrum in the range of 200–700 nm. After Ru
impregnation, the presence of KRuO4 can be observed by an
absorbance at 300–330 nm in Ru-FAU-2. Nitrogen sorption
data show that the zeolite pore space at high Ru loadings is
greatly diminished. Ru-FAU-2 has a void volume of 0.231 cc
g21, similar to the value for K-X of 0.253 cc g21 (void volumes
are reported as cc of liquid nitrogen adsorbed per gram of dry
solid). However, Ru-FAU-10 has a void volume of only 0.030
cc g21. The bulk composition of Ru-FAU-2 and Ru-FAU-10 are
0.18 and 0.80 Ru/Si (mol/mol), respectively. XPS of the
superficial area (surface plus several cages in depth) of each Ru-
FAU sample showed that the composition near the surface of
the zeolites was 0.173 and 18.33 Ru/Si, for Ru-FAU-2 and Ru-
FAU-10, respectively. Thus, Ru-FAU-10 has significantly more
Ru in the superficial region and suggests that some Ru could be
on the exterior surface of the zeolite. This result is consistent
with the reactivity observed from Ru-FAU-10 with 1-pyr-
enemethanol.
Notes and references
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2
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added to observe the activity of the RuO42 species. In order to
have a true catalyst, co-oxidants could be added or cyclic
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Table 2 Oxidation of 1-pyrenemethanol 3 over Ru-containing materials (1
mmol 3+1.5 mmol Ru, in 5 mL DCE)
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Reaction time/h
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24
72
120
24
89
100
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1
45
2
K-X
24
24
24
0
100
0
2
CPG-P+RuO4
CPG-240
a Relative to moles of 3.
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