Creation of a monomeric ruthenium species on the surface of micro-size copper hydrogen phosphate: An active heterogeneous catalyst for selective aerobic oxidation of alcohols

Article abstract of DOI:10.1002/adsc.200700187

A new micro-size copper hydrogen phosphate (CHP) synthesized by the emulsion method combined with a monomeric ruthenium species was found to be a very effective catalyst for the selective oxidation of alcohols. Several kinds of alcohols were transformed into the corresponding aldehydes or ketones over the RuCHP catalyst by oxygen under very mild conditions. The results showed that the CHP material was perfect as a catalyst support due to its high ion-exchange ability and adsorption capacity.

Full text of DOI:10.1002/adsc.200700187

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DOI: 10.1002/adsc.200700187
Creation of a Monomeric Ruthenium Species on the Surface of
Micro-Size Copper Hydrogen Phosphate: An Active
Heterogeneous Catalyst for Selective Aerobic Oxidation of Alcohols
Junhua Liu,a,b Fang Wang,a,b Kunpeng Sun,a and Xianlun Xua,*
a
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese
Academy of Sciences, Lanzhou 730000, Peopleꢀs Republic of China
Fax : (+86)-931-827-7088; e-mail: xlxu@lzb.ac.cn
Graduate School, Chinese Academy of Sciences, Beijing 100049, Peopleꢀs Republic of China
b
Received: April 16, 2007; Revised: August 17, 2007
Abstract: A new micro-size copper hydrogen phos-
phate (CHP) synthesized by the emulsion method
combined with a monomeric ruthenium species was
found to be a very effective catalyst for the selec-
tive oxidation of alcohols. Several kinds of alcohols
were transformed into the corresponding aldehydes
or ketones over the RuCHP catalyst by oxygen
under very mild conditions. The results showed that
the CHP material was perfect as a catalyst support
due to its high ion-exchange ability and adsorption
capacity.
Various kinds of transition metal cations, which
have a high potentiality for functioning as the catalyt-
ic active center, can be readily accommodated into
some ores, such as hydrotalcite, apatite, hydroxyapa-
tite and montmorillonite, based on their large cation
exchange ability.[5] Also, use of hydroxyapatite as cat-
alyst support is motivated by the following advantag-
es:[6] (1) well-defined monomeric active species can
be immobilized on the surface, on the basis of high
ion-exchange ability and adsorption capacity; (2) the
non-porous structure can help to overcome the prob-
lems of mass transfer limitation; (3) the weak acid-
base properties prohibit side reactions induced by the
support itself. What interests us is whether some
kinds of metal phosphates have similar or better cata-
lytic performances as the above samples; meanwhile,
ores used as catalyst carriers were all prepared by
conventional methods in those articles published so
Keywords: alcohols; aldehydes; copper hydrogen
phosphate; ketones; oxidation; ruthenium
The aerobic oxidation of alcohols to aldehydes and far,[5] and if the sizes of the phosphates material were
ketones is a fundamental chemical transformation for measured in micro or nano size range, what about
the production of a large variety of important inter- their catalytic performance. Thus, a micro-size copper
mediates and fine chemical products.[1] However, hydrogen phosphate (CuHPO4; CHP)[7] prepared by
most of these oxidants are toxic, hazardous, or are re- the emulsion method was reported in this article and
quired in large excess. From an economical and envi- if monomeric Ru cationic species could be uniformly
ronmental viewpoint, catalytic oxidations are particu- fixed on the surface of it, then a Ru3+-exchanged
larly interesting. Recently, much attention has been copper hydrogen phosphate (RuCHP) compound
paid to the development of metal catalysts employing would be prepared. Although CHP has a lower sur-
O2 (or air) or hydrogen peroxide as the ultimate stoi- face area (only 7.09 m2 gÀ1) than the other metal phos-
chiometric oxidant. These attractive catalytic systems phates (e.g., HAP), and there are no micropores or
using cheap and readily available reactants display mesopores, the RuCHP catalyst showed unique cata-
high atom efficiency and water is the only chemical lytic performance for the oxidation of alcohols using
by-product of the oxidation reactions.[2] Due to the molecular oxygen. To the best of our knowledge, this
disadvantages of homogeneous metal catalysts, cur- is the first report of the preparation of a new structur-
rent research has been focused on the development of al copper hydrogen phosphate by the emulsion
efficient heterogeneous catalysts possessing superior method and utilization of the material as catalyst pre-
activity as well as high reusability. Many reports on cusor for oxidation by ion-exchange with ruthenium.
heterogeneous palladium catalysts have been pub- This method is fairly mild and easy to handle.
lished up to now,[3] and a few examples of solid-sup-
ported ruthenium catalysts are available.[4]
Figure 1 (a) shows the FT-IR spectra for the cal-
cined copper hydrogen phosphate, the typical peaks
for phosphate were found at ca. 615 cmÀ1 (P=O bend-
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Junhua Liu et al.
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Table 1. The structural parameters of the RuCHP catalyst.
Shell
Coordination
number
Interatomic
distance [Š]
Ds[a] [Š2]
À
Ru O(1)
4.0
2.0
2.0
1.0
1.98
2.28
2.60
2.32
0.0068
0.0077
0.0095
0.0085
À
Ru O(2)
À
Ru O(3)
À
Ru Cl
[a]
Debye–Waller factor.
Figure 1. (a) FT-IR spectra of the CHP; (b) XRD of the
CHP (as prepared, calcined) and RuCHP.
À1
3À
ing), 758 cmÀ1 [P O(H) streching], 956 cm (PO4
,
À
À1
3À
À
À
P O streching), 1098 cm (PO4 , P O streching),
3439 cmÀ1 (hydrogen bonded OH).[8] The XRD peak
positions of RuCHP were similar to those of the
parent CHP, while the strengths of the peaks of
RuCHP were slightly weaker than those of CHP
[Figure 1 (b)]. From the elemental analysis (XRF),
the (Ru+Cu)/P ratio of the RuCHP was estimated to
be 0.85 (Ru content: 6.1 wt%, 0.59 mmolgÀ1), which
was lower than that of Cu/P (1:1) in the parent CHP.
Inductively coupled plasma (ICP) analysis revealed
that copper was present in the filtrate after the ruthe-
nium loading. XRF, EDS elemental analysis and ICP
analysis all testified that ruthenium had exchanged
with copper on the surface of CHP to give birth to
RuCHP. The presence of chlorine was confirmed by
XPS and XRF analyses, the atomic ratio of Ru to Cl
was 1:1. The structural analysis results of RuCHP cat-
alyst obtained from EXAFS spectra are shown in
Figure 2. (a) Fourier transform (FT) magnitude of k2-weight-
ed EXAFS of RuCHP; (b) inverse FT of peaks with the
0.9<R/Š<3.0 range of part (a); (c) a proposed structure of
the RuCHP catalyst.
Table 1 and Figure 2. The Fourier-transformed (FT) curring between 0.9 and 2.8 Š could be attributed to
spectrum of RuCHP was different from those of the oxygen nearest neighbours, the inverse FT of main
À
reference samples RuCl3, RuO2, the lack of peaks peaks was well fitted by the use of three Ru O and
À
À
above 3.0 Š indicated no Ru Ru bond in this com- single Ru Cl shells. On the basis of the above data, a
plex, while it was clearly visible in RuO2, and possible structure of this catalyst was proposed. The
RuCl3,[9d] hence, monomeric Ru seems to be highly sheet morphologies of the CHP and RuCHP were ob-
dispersed in CHP. Also, this conclusion was supported served by SEM, compared with the parent CHP, the
by fitting of the data (Table 1), scattering events oc- crystal phase of the RuCHP had a slight breakage
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Monomeric Ruthenium Species on the Surface of Micro-Size Copper Hydrogen Phosphate
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Figure 3. SEM images of the (top) CHP (calcined) and
(bottom) RuCHP.
(Figure 3), HRTEM testified that it had no micro-
pores or mesopores, also, no Ru3+ particles were
found in a great extent of the CHP surface [Figure.4
ACHTREUNG
(top)], but adsorption of Ru3+ ions (and ClÀ ions) also
Figure 4. HRTEM images of the CHP (top) no Ru3+ parti-
cles; (bottom) with Ru3+ particles.
was found [Figure 4 (bottom)], this means that the
Cu2+ ion exchange with Ru3+ is not absolute. In fact,
the nature of the interaction between phosphates and
metal ions in aqueous solution is composed by three
steps: surface adsorption, ion exchange between Cu2+ periment the oxidation of benzyl alcohol was also per-
and metal ions involving diffusion into the solid and formed to give a 92% yield of benzaldehyde; a TON
dissolution of CHP and precipitation of a new phase as high as 1040 could be achieved (entry 3). Although
(or phases), these processes often act together.[9d]
a long reaction time was demanded, primary aliphatic
The catalytic activity of the copper hydrogen phos- alcohols were smoothly oxidized to aldehydes in high
phate-supported ruthenium catalyst was examined for yields (entries 11 and 12). Moreover, this catalyst
the catalytic aerobic oxidation of alcohols at 808C system was applicable for the oxidation of heterocy-
under O2 bubbling conditions; representative results clic alcohols, for example, furan-2-methanol and pyri-
are shown in Table 2. All the benzylic and allylic alco- dine-2-methanol gave the corresponding aldehydes in
hols showed high reactivity for the oxidative dehydro- high yields (entries 13 and 14), it was worthy to men-
genation (entries 1, 3–10), they were converted to the tion that cyclohexanol and cyclooctanol were effec-
corresponding aldehydes or ketones in 100% conver- tively oxidized to the corresponding cycloalkanones
sions and 92–99% yields. In an enlarged activity ex- in 18 h (entries 15 and 16). Also, bubbling oxygen was
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Junhua Liu et al.
Yield [%][b]
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Table 2. Oxidation of alcohols with hydrogen peroxide catalyzed by RuCHP with molecular oxygen.[a]
Entry
Time (h)
Substrate
Product
Conversion [%][b]
1
4
10
48
100
2
92
>99
2
92
2[c]
3[d]
4
5
100
>99
5
6
5
5
100
100
>99
>99
7
1
1
2
4
100
100
96
98
96
92
93
8
9
10
97
11
12
12
10
95
97
95
97
13
14
14
18
100
100
>99
>99
15
18
18
92
97
92
97
16
[a]
Alcohol (10 mmol), RuCHP (0.2 g), toluene (5 mL), 808C, O2 bubbling.
Determined by GC.
Ar atmosphere.
[b]
[c]
[d]
An enlarged activity experiment: alcohol (20 mmol), RuCHP (0.03 g), toluene (10 mL).
necessary for the oxidation of alcohols under air con- reaction temperature, but further treatment of the fil-
ditions in place of pure O2, the above oxidations pro- trate under similar reaction conditions did not afford
ceeded smoothly but slowly, a 72% yield of benzalde- to any products. The oxidation of cinnamyl alcohol to
hyde was obtained (the conditions were the same as cinnamyl aldehyde by CHP without ruthenium was
entry 1, Table 2), while the reaction almost could not carried under the same conditions as entry 8, but only
proceed in the presence of an argon atmosphere a trace of product was obtained, it was concluded that
(entry 2). Furthermore, the recycled catalyst could be CHP itself had a slight oxidation function.
used for oxidation of benzyl alcohol without changes
During the reaction courses, we noticed that: (1)
in conversion and selectivity values, it gave a 98% when the actual oxygen concentration was monitored,
yield for the second use and a 97% yield for the third controlled by the rate of oxygen supply to the catalyst
use. It was confirmed by induced coupled plasma surface, it had a strong influence on the overall reac-
techniques (ICP) that the Ru content of recycled tion rate; (2) one mole of H2O was produced for
RuCHP catalyst was the same as that of the fresh one every mole of benzaldehyde formation; (3) XPS re-
and no Ru was detected in the filtrate. Also, the cata- vealed that no reduction of Ru3+ species to Ru2+ or
lyst was filtered off after ca. 50% conversion at the Ru(0) took place after oxidation of benzyl alcohol;
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Monomeric Ruthenium Species on the Surface of Micro-Size Copper Hydrogen Phosphate
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(4) CHP itself could not effect the oxidation reaction Oxidation Reactions of Alcohols with Oxygen
by oxygen. On the basis of literature data[4b,9] and the
All reactions were performed in a round-bottom one-neck
above experimental measurements, a possible catalyt-
glass flask equipped with a reflux condenser, a magnetic stir-
ic cycle for this reaction system using RuCHP was
proposed: the oxidation was initiated by a ligand ex-
change between an alcohol and a Cl species of the
RuCHP to give a Ru alcoholate species, which then
underwent b-hydride elimination to produce the car-
bonyl compound and a hydrido-ruthenium species,
subsequent the catalyst was regenerated by molecular
oxygen to close the catalytic cycle.
rer and a gas inlet allowing us to bubble a flow of oxygen
into the reaction mixture (25 mLminÀ1). A typical proce-
dure was as follows: a mixture of benzyl alcohol (10 mmol),
RuCHP (0.2 g) in a solvent (5 mL toluene) was charged in
the reactor and saturated with oxygen at room temperature
for 5 min. Then the reactor was placed into the oil bath pre-
heated to 808C to start the reaction, the resulting mixture
was stirred under bubbling oxygen for 4 h. The products
were identified by GC using the authentic samples of alde-
hydes or ketones.
In conclusion, a simple and convenient route for
the synthesis of micro-size copper hydrogen phos-
phate by a non-ionic surfactant emulsion method was
achieved. Then an RuCHP catalyst with monomeric
Ru species was prepared by ion-exchange. This cata-
lyst could act as an effective heterogeneous catalyst
for the oxidation of various alcohols (benzylic, allylic
and cycloaliphatic alcohols). The catalyst was recycla-
ble and no Ru leaching was observed during the oxi-
dations. The present simple and mild method to pre-
pare micro-size copper hydrogen phosphate can be
applied to other metal compound systems, some other
samples measured in the micro- or nano-size range
have been prepared in our group. Also, this simple
design strategy for CHP-bound transition metal cata-
lysts can be further applied to other systems, we antic-
ipate that these novel catalysts also will exhibit out-
standing catalytic performances in a wide range of
functional transformations in organic synthesis, it will
be reported in our subsequent papers.
Acknowledgements
The authors would like to thank Professor B.X. Lin (Peking
University) for the determining the fabric of the CHP, Profes-
sor K. Q. Lu (Institute of Physics, Chinese Academy of Sci-
ences) for the EXAFS measurements and Professor S. Q. Wei
(National Synchrotron Radiation Laboratory, University of
Science & Technology of China) for the EXAFS data analy-
sis.
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Experimental Section
Preparation of Micro-Size Copper Hydrogen
Phosphate (CuHPO4) by a Surfactant Emulsion
Method[10]
Aqueous solutions of copper nitrate and phosphoric acid
were made by dissolving 6.04 g CuACHTRE(UGN NO3)2·3H2O (0.025M)
and 1.8 mL 85% H3PO4 in 8 mL deionized water. 4 g surfac-
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mixed according to the volume ratio 1:10, the pH of the
emulsion was adjusted to 7 by adding concentrated ammoni-
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was aged at 258C for 24 h. The mixture was centrifuged and
washed with absolute ethanol and water. After drying over-
night in a vacuum oven at 808C, the product was calcined at
5008C for 6 h to obtain grey CHP precursor powders. The
CHP (1.0 g) was stirred with 75 mL of a 2.0”10À2 M aqueous
RuCl3 solution at 258C for 24 h. The obtained slurry was fil-
tered, washed with deionized water, and dried overnight in
a vacuum oven at 808C yielding the RuCHP as a dark
brown powder.
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Junhua Liu et al.
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