Selective transfer hydrogenation of carbonyl compounds by ruthenium nanoclusters supported on alkali-exchanged zeolite beta

Article abstract of DOI:10.1002/adsc.200505497

Selective transfer hydrogenation of aromatic ketones and β-keto esters to the corresponding alcohols was achieved by using ruthenium nanoclusters supported on alkali-exchanged zeolite beta catalyst. The high activity and selectivity of the catalyst is due to the presence of highly dispersed ruthenium clusters in combination with the large number of Bronsted acidic sites of zeolite.

Full text of DOI:10.1002/adsc.200505497

FULL PAPERS
DOI: 10.1002/adsc.200505497
Selective Transfer Hydrogenation of Carbonyl Compounds by
Ruthenium Nanoclusters Supported on Alkali-Exchanged Zeolite
Beta
M. Lakshmi Kantam,a,* B. Purna Chandra Rao,a B. M. Choudary,b,*
and B. Sreedhara
a
Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad 500007, India
Fax: (+91)-40-2719-3510; e-mail: mlakshmi@iict.res.in
Ogene Systems (I) Pvt. Ltd., # 11-6-56, GSR Estates, Moosapet, Hyderabad 500037, India
b
Fax: (+91)-40-2377-5566; e-mail: bmchoudary@gmail.com
Received: December 30, 2005; Revised: June 24, 2006; Accepted: July 11, 2006
Abstract: Selective transfer hydrogenation of aro- clusters in combination with the large number of
matic ketones and b-keto esters to the corresponding Brønsted acidic sites of zeolite.
alcohols was achieved by using ruthenium nanoclus-
ters supported on alkali-exchanged zeolite beta cata-
lyst. The high activity and selectivity of the catalyst Keywords: chemoselectivity; nanoclusters; rutheni-
is due to the presence of highly dispersed ruthenium um; transfer hydrogenation; zeolite beta
Introduction
asymmetric transfer hydrogenation catalysts as well as
the use of these silica-supported systems in batch and
Selective hydrogenation of carbonyl compounds to al- flow reactors.[6] Ley and co-workers reported the het-
cohols is a synthetically important transformation erogeneous transfer hydrogenation of aryl ketones
both in the laboratory and in industry. Many homoge- using a recyclable [Pd(0)EnCat] catalyst.[7]
neous or heterogeneous catalysts have been studied
Zeolite-supported metal clusters are considered to
for this transformation using molecular hydrogen as be versatile heterogeneous catalysts because they
hydrogen source.[1] Recently most of the studies were offer conceptual links between molecular and surface
concentrated on the transfer hydrogenation reaction catalysis[8] and provide the advantages of the two
because it is operationally simple and can avoid the classes of catalysts. Zeolites and Al2O3-supported
use of molecular hydrogen.[2] Both mono- and poly- transition metal catalysts are excellent for selective
nuclear Ru(II), Rh(I), and Ir(I) complexes have been hydrogenations[9] of various carbonyl compounds. Re-
successfully employed as catalysts for homogeneous cently Ru-containing BEA zeolites were reported for
asymmetric transfer hydrogenations.[3] Among these the hydrogenation of a conjugated cyclic keto-enol[10]
ruthenium complex-catalyzed transfer hydrogenations and it was shown that the acid properties of the zeo-
have proved to be particularly reliable.[3b–d] However, lite and dispersion of the ruthenium particles were
the application of such catalysts is limited, partly due important for selective hydrogenation. It is also found
to the problems of separation and recycling of the cat- that the presence of acidic conditions enhances the
alysts. To overcome such drawbacks, immobilization activity in the hydrogenation of b-keto esters.[11] In
of homogeneous catalysts on insoluble supports has this paper, we report the selective transfer hydrogena-
received considerable interest in recent years as it tion of aromatic ketones and b-keto esters by using
simplifies the separation of the catalysts from the re- ruthenium nanoclusters supported on alkali-ex-
action mixtures and allows the efficient recovery and changed zeolite beta.
reuse of catalysts.[4] Examples of immobilized transfer
hydrogenation catalysts are still rare. There are re-
ports on transfer hydrogenations using catalysts non-
covalently or covalently linked to the support.[5] In
these systems, the catalysts are deactivated or the re-
actions are extremely slow. Later van Leeuwen and
co-workers reported the solid-phase synthesis of
1970
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2006, 348, 1970 – 1976

Selective Transfer Hydrogenation of Carbonyl Compounds
FULL PAPERS
Results and Discussion
Synthesis and Characterization of Ruthenium
Nanoclusters Supported on Cesium Zeolite Beta
Catalysts
The selection of zeolite beta (b) for the incorporation
of ruthenium nanoclusters was based on the fact that
this is a large-pore, tridimensional zeolite and upon
exchange with cesium, the basicity of the zeolite in-
creases to stabilize the supported metal species by
generating stronger metal-support interactions, a con-
sequence of which is a higher metal dispersion after
reduction.[12] We have incorporated the ruthenium
nanoclusters into cesium zeolite beta (Cs-b) by ion
exchange (IE) and impregnation (IMP) methods
using the precursors RuCl3, [Ru(NH3)6]Cl3 and
G
Ru3(CO)12. The chemical composition and physical
characteristics of these samples are given in Table 1.
The surface area and pore volume values decrease
slightly for Ru Cs-b
bonyl) but largely for Ru Cs-b
ACHTREUNG
AHCTREUNG
tion (XRD) patterns recorded for different types of
ruthenium nanoclusters supported zeolite beta cata-
lysts (Figure 1) show the typical zeolite beta structure
even after incorporation of the ruthenium nanoclus-
ters. Also, a new diffraction line at around 43o, was
observed in the XRD patterns of Ru Cs-b
ACHTREUNG
Cs-b
C
ACHTREUNG
Figure 1. Powder XRD patterns of different zeolite beta cat-
alysts (* indicates peaks due to Ru crystallites).
carbonyl absorption region, which is similar to the two different Ru species are present. Deconvolution
earlier reported MCM-41 supported platinum carbon- shows the low (at 281.8, 285.8) and high (at 283.1,
yl cluster.[14] These data reveal that free ruthenium 287.8) eV binding energies of ruthenium with an in-
clusters were dispersed in the Ru Cs-b(IMP-carbonyl) tensity ratio of 3:2. These BE components correspond
sample.
The binding energies (BE) of Ru (3d5/2) in Ru Cs-b- dation of the surface of the free ruthenium clusters
(IE-Cl), Ru Cs-b(IE-amine) and Ru Cs-b(IMP-Cl) upon exposure to air which is in agreement to the ear-
were close to 280.0 eV (Table 1), indicating the pres- lier report.[15] Transmission electron micrograph
ence of metallic ruthenium. The BE of Ru Cs-b(IMP- (TEM) images of Ru Cs-b(IE-Cl), Ru Cs-b(IE-
carbonyl) shows broad and small valleys between the amine) and Ru Cs-b(IMP-Cl) samples showed ag-
to the RuO2 cluster species, probably formed by oxi-
A
N
ACHTREUNG
A
ACHTREUNG
AHCTREUNG
spin-orbit components, which clearly indicates that glomerated ruthenium metal particles whereas highly
Table 1. Chemical composition, textural characteristics and binding energies of zeolite beta catalysts.
Catalyst
Precursor
% Ru[a]
(w/w)
Pore volume
[cm3 gÀ1]
Surface area [m2 gÀ1]
Binding energies
Ru 3d5/2 [eV]
Hb
Cs-b
Ru Cs-b
Ru Cs-b
Ru Cs-b
-
-
-
-
0.962
1.000
0.462
0.899
0.618
0.589
463.1
436.4
267.1
486.7
389.2
389.6
-
-
G
RuCl3
Ru
RuCl3
Ru3(CO)12
1.80
0.62
5.55
5.50
280.8
280.5
280.4
G
[(NH3)6]Cl3
U
Ru Cs-b(IMP-carbonyl)
281.8, 283.1, 285.8, 287.8
[a]
Analyzed by ICP-AES.
Adv. Synth. Catal. 2006, 348, 1970 – 1976
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.asc.wiley-vch.de
1971

M. Lakshmi Kantam et al.
FULL PAPERS
dispersed uniform ruthenium metal particles were ob- sorption and TEM measurements. The number of
served in Ru Cs-b(IMP-carbonyl) (Figure 2 a). acidic sites was determined by means of NH3-TPD
Hydrogen chemisorption was employed to estimate method in the range of 150–6508C and these results
the dispersion of ruthenium and the cluster size after are condensed in Table 2. It is observed that the
reduction of the samples. The comparison of the re- amount of NH3 adsorbed on the Cs-b is decreased
sults of hydrogen adsorption is presented in Table 2. with a concomitant decrease in the maximum of the
The metal dispersion varies in the order, Ru Cs-b- desorption temperature and therefore the acidic
A
G
ACHTREUNG
A
ACHTREUNG
pregnation with Ru3(CO)12 adsorb large amounts of than in Cs-b and this may be due to the partial shield-
hydrogen and show high metal dispersion and the ing of acidic sites by ruthenium clusters present out-
ruthenium cluster size is less than 5 nm. The samples side the pores. The numbers of acidic sites have in-
prepared by ion exchange of ruthenium hexammine creased slightly for Ru Cs-b(IMP-Cl) and largely for
N
chloride also adsorb considerable amounts of hydro- Ru Cs-b(IMP-carbonyl). The increase in the number
gen and the dispersion is also high compared to the of acidic sites in Ru Cs-b(IMP-carbonyl) may be due
samples prepared by ion exchange or impregnation to the presence of electron-deficient sites which are
with RuCl3. This is in accord with the recent report formed by the interaction of small ruthenium clusters
that the hexammine chloride complex allows a better with electron acceptor sites of zeolite.[18]
penetration into the mesopores of MCM-41 than
RuCl3 and ruthenium acetylacetonate.[16] The decrease
in metal dispersion by using the RuCl3 precursor Survey of the Reaction Parameters
might be a consequence of the agglomeration of un-
dissolved RuCl3 crystals on the surface of the Cs-b Various ruthenium nanoclusters supported catalysts
during the ion-exchange. The subsequent reduction were prepared from different Ru precursors, viz.
results in the formation of larger ruthenium crystalli- RuCl3, [Ru(NH3)6]Cl3 and Ru3(CO)12, and tested for
G
tes most likely located on the outer surface of Cs-b the transfer hydrogenation of acetophenone (Table 3).
particles. The results of the hydrogen adsorption ex- The optimized reaction conditions are: 2-propanol as
periments were confirmed by powder XRD, N2 ad- hydrogen donor, 10 mol% of KOH and catalyst con-
Figure 2. TEM images of (a) fresh and (b) used catalyst.
Table 2. Metal dispersions, cluster sizes and acid properties of different ruthenium containing zeolite beta catalysts.
Catalyst
Hydrogen adsorbed [mmol gÀ1] Dispersion [%] Cluster size [nm] Acidity TPAD [mmolgÀ1
]
Hb
Cs-b
Ru Cs-b
Ru Cs-b
Ru Cs-b
-
-
0.8
2.2
1.9
-
-
-
-
0.859
0.508
0.322
0.458
0.567
1.998
G
0.96
7.33
0.70
22.15
103.69
13.56
142.28
4.49
ACHTREUNG
ACHTREUNG
Ru Cs-b(IMP-carbonyl) 60.3
1972
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© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2006, 348, 1970 – 1976

Selective Transfer Hydrogenation of Carbonyl Compounds
FULL PAPERS
Table 3. Transfer hydrogenation of acetophenone with dif-
Table 4. Transfer hydrogenations of various aryl ketones
with 2-propanol over Ru Cs-b(IMP-carbonyl) catalyst.[a]
ferent ruthenium catalysts.[a]
Entry Catalyst
Time [h] Conversion[b] [%]
1
2
3
4
5
6
5
Ru3(CO)12
H b
Cs b
24
24
24
24
24
24
25
0
0
0
0
Ru Cs-b
Ru Cs-b
Ru Cs-b
C
14
100
Ru Cs-b(IMP-carbonyl) 20
[a]
Reaction conditions: acetophenone (1 mmol), 2-propanol
(5 mL), KOH in 2-propanol (0.1 mol), catalyst (2.5
mol% of Ru), refluxed at 828C.
[b]
GC yields.
taining 2.5 mol% of Ru at 828C. The Ru Cs-b
and Ru Cs-b(IE-amine) catalysts do not give any
product even after 24 h. The catalyst Ru Cs-b(IMP-
A
ACHTREUNG
AHCTREUNG
Cl) has shown 14% conversion after 24 h while the
catalyst Ru Cs-b(IMP-carbonyl) has shown complete
conversion within 20 h. Even the homogeneous cata-
lyst Ru3(CO)12 containing the same amount of Ru af-
forded 25% conversion after 24 h. Furthermore, there
is no reaction by using Ru-free H-b or Cs-b catalysts.
The reason for high catalytic activity of Ru Cs-
b(IMP-carbonyl) catalyst is probably the high disper-
sion of ruthenium nanoclusters in combination with a
large amount of acidic sites (Table 2).[10]
Scope of the Reaction
The scope of the catalyst was established by using a
wide range of aromatic ketones and in all cases the
reaction was completed with in 20–48 h with exceed-
ingly high yields (Table 4). As illustrated in Table 4,
selective reduction of 4-methoxyacetophenone and 4-
bromoacetophenone (entries 5 and 6) is possible to
afford the corresponding alcohols in excellent yields
albeit with longer reaction times. It is also important
[a]
Reaction conditions: substrate (1 mmol), 2-propanol (5 mL),
KOH in 2-propanol (0.1 mol), catalyst (50 mg, 2.5 mol%
of Ru), refluxed at 828C.
[b]
GC yields
[c]
Yields in parenthesis are for three successive cycles.
Isolated yields.
[d]
to note that the high yields were obtained in the re- Selective Transfer Hydrogenations
duction of 4-isobutylacetophenone, 6-methoxy-2-ace-
tonapthone, 2,4-dichloro-5-fluoroacetophenone (en- The active catalyst in the transfer hydrogenation of
tries 4, 7 and 14) and the corresponding alcohols are aromatic ketones was also tested for the chemoselec-
intermediates for ibuprofen, naproxen and ciprofloxa- tive hydrogenation of various substrates with 10
cin, respectively. Moreover, the reduction of the com- mol% KOH in 2-propanol and the results are pre-
pounds containing substitutents present either on the sented in Table 5. Citronellal, a non-conjugated unsa-
aromatic ring or in the a- position (entries 2–4 and 8– turated aldehyde, is selectively reduced to the corre-
13) reveals that steric hindrance is less effective in the sponding unsaturated alcohol (citronellol). This is due
present reaction.
to the high dispersion of ruthenium which selectively
Adv. Synth. Catal. 2006, 348, 1970 – 1976
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.asc.wiley-vch.de
1973

M. Lakshmi Kantam et al.
FULL PAPERS
Table 5. Selective transfer hydrogenations with 2-propanol
over Ru Cs-b(IMP-carbonyl) catalyst.[a]
Table 6. Analysis of Ru in Ru Cs b(IMP-carbonyl) catalyst
in transfer hydrogenation of acetophenone.[a]
Reusability
Ru[b] [mmolg]À1
Yield[c] [%]
Fresh reaction
1st recycle
0.544
0.543
0.542
0.538
0.544
-
99
99
97
2nd recycle
3rd recycle
Hot filtered catalyst
Filtrate
96
39[d]
39[e]
[a]
Reaction conditions: acetophenone (1 mmol), 2-propanol
(5 mL), KOH in 2-propanol (0.1 mol), catalyst, refluxed
at 828CC.
Analyzed by ICP-AES.
GC yields.
After 8 h.
Reaction conducted with the filtrate.
[b]
[c]
[d]
[e]
ued with the filtrate for additional 12 h. No further
product formation was observed from GC analysis.
Moreover, the absence of Ru in the filtrate was deter-
mined by ICP-AES.
XPS analysis of the used Ru Cs-b(IMP-carbonyl)
catalyst shows low (at 282.0, 285.8 eV) and high (at
283.2, 288.0 eV) binding energy values of ruthenium
similar to the binding energies observed for the fresh
catalyst, which corresponds to the RuO2 cluster spe-
cies. No metallic ruthenium (~280.0) was observed.
[a]
Reaction conditions: substrate (1 mmol), 2-propanol (5 mL),
KOH in 2-propanol (0.1 mol), catalyst (50 mg, 2.5 mol%
of Ru), refluxed at 828C.
GC yields.
Isolated yields.
[b]
[c]
hydrogenates the carbonyl group.[19] Transfer hydroge- The TEM image of the used catalyst (Figure 2 b) also
nation of b-keto esters (entries 2–5) afforded com- shows highly dispersed ruthenium particles (<2 nm).
plete conversion to b-hydroxy esters. The high activity These results demonstrate that the used catalyst is the
can be attributed by the fact that the Brønsted acidic same as the fresh catalyst and highly dispersed ruthe-
sites of zeolite protonate the carbonyl group to facili- nium nanoclusters retain their properties even after
tate the hydride insertion.[11]
completion of the reaction and the catalyst is active
for several cycles.
Reusability
Conclusions
The reusability is an important point concerning the
use of the catalyst both for industrial and pharma- Ruthenium nanoclusters supported on alkali zeolite
ceutical applications. As the solid catalyst can be re- beta catalysts were prepared by using ion exchange
covered easily from the reaction mixture, we exam- and impregnation methods with various precursors.
ined the reusability of the catalyst in the transfer hy- Highly dispersed ruthenium nanoclusters were ob-
drogenation of acetophenone. After the first run, the tained using the Ru3(CO)12 precursor and the number
catalyst was separated by filtration, washed with 2- of acidic sites increased four-fold with respect to the
propanol and dried in an oven. Table 6 presents the parent support (Cs-b). Ru Cs-b(IMP-carbonyl)
results of recycling of the catalytic system. Four cycles showed high activity in the transfer hydrogenation of
were performed without any significant loss of activi- aromatic ketones and chemoselective hydrogenation
ty. ICP-AES analysis of the used Ru Cs-b(IMP-car- of citronellal and b-keto esters.
bonyl) catalyst showed 1.1% of ruthenium leaching
after the 4th cycle.
The ruthenium leaching was also studied as follows.
The transfer hydrogenation of acetophenone using
Experimental Section
Ru Cs-b(IMP-carbonyl) catalyst was stopped after
8 h, and the conversion was 39%. At this juncture the
Preparation of Supports
catalyst was separated from the reaction mixture at
the reaction temperature and the reaction was contin- (Zeolyst International) was exchanged twice with a 0.5M
The parent zeolite beta (b) having an SiO2/Al2O3 ratio of 25
1974
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© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2006, 348, 1970 – 1976

Selective Transfer Hydrogenation of Carbonyl Compounds
FULL PAPERS
NaNO3 solution (70 mL/g, 808C, 2 h, under stirring). After
washing with water, the solid (Na-b) was recovered by cen-
trifugation and dried overnight. Cesium zeolite beta (Cs-b)
was obtained by exchanging Na-b with a 0.5M solution of
cesium acetate (Aldrich 99.9%). The exchange procedure
was the same as that mentioned above. The Cs-b was then
calcined in a flow of air at 4508C for 2 h.
Ruthenium was introduced in the so-obtained Cs-b zeolite
by ion exchange and impregnation methods. The ion ex-
changed samples were obtained by stirring Cs-b in an aque-
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ous solutions of RuCl3 or [Ru(NH3)6]Cl3 at 708C for 24 h.
N
The solids were recovered by filtration and washed thor-
oughly with distilled water, dried and calcined in a flow of
air at 4508C for 2 h. These samples were then reduced at
4508C for 6 h in a flow of H2 (30 mLminÀ1). The impregnat-
ed samples were also prepared by the same procedure and
water was evaporated in a rotary vapor. The sample was cal-
cined at 4508C for 2 h and then reduced at 4508C for 6 h in
a flow of H2 (30 mLminÀ1). The impregnated samples were
also prepared by stirring Cs-b in an solution of n-pentane
containing Ru3(CO)12 at 308C for 24 h and then dried in a
rotary vapor.
Reaction Conditions
A reaction mixture containing 1 mmol of the substrate and
5 mL of 2-propanol was placed in a two-neck, round-bot-
tomed flask equipped with a septum port, reflux condenser
and a guard tube. To this mixture were added 50 mg of
freshly vacuum-dried catalyst. The transfer hydrogenation
reaction was carried under reflux at 828C. Aliquots were re-
moved at different reaction times and products were ana-
lyzed by gas chromatography (Shimadzu 2010) fitted with
an OV-17 column. The identity of the products was con-
firmed by NMR and MS analyses. The reusability of the cat-
alyst was tested after washing the used catalyst with 2-prop-
anol followed by drying at 1008C.
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Acknowledgements
We wish to thank the CSIR for financial support under the
Task Force Project COR-0003. B. P. C. R. thanks the Council
of Scientific and Industrial Research (CSIR), India, for the
award ofSenior Research Fellowship.
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