An efficient heterogeneous catalytic system for chemoselective hydrogenation of unsaturated ketones in aqueous medium

Article abstract of DOI:10.1016/j.poly.2010.09.006

A highly chemoselective and green heterogeneous catalytic system of immobilized Ru(II)-phenanthroline complexes on amino functionalised MCM-41 material for the chemoselective hydrogenation of unsaturated ketones to unsaturated alcohols is demonstrated using water as a solvent. The XRD and FTIR spectra show the highly ordered hexagonal nature of the MCM-41, even after encapsulation of the ruthenium complex. The complex retains its configuration after anchoring, as was confirmed by FTIR and UV-Vis analysis. The detailed reaction parametric effect was studied for the hydrogenation of 3-methylpent-3-en-2-one to achieve complete conversion up to >99% chemoselectivity of 3-methylpent-3-en-2-ol. The anchored heterogeneous catalysts were recycled effectively and reused five times with marginal changes in activity and selectivity. The use of water as a solvent not only afforded high activity for the hydrogenation reaction compared to organic solvents, but also afforded a green process.

Full text of DOI:10.1016/j.poly.2010.09.006

Polyhedron 29 (2010) 3262–3268
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
An efficient heterogeneous catalytic system for chemoselective hydrogenation
of unsaturated ketones in aqueous medium
a
c
b
Amit Deshmukh a,b, , Anil Kinage , Rajiv Kumar , Reinout Meijboom
⇑
a Catalysis Division, National Chemical Laboratory, Pune 411 008, India
b Chemistry Department, University of Johannesburg, Auckland Park, Johannesburg, South Africa
c Tata Chemicals Ltd., Pashan, Pune, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 July 2010
Accepted 2 September 2010
Available online 17 September 2010
A highly chemoselective and green heterogeneous catalytic system of immobilized Ru(II)–phenanthroline
complexes on amino functionalised MCM-41 material for the chemoselective hydrogenation of unsatu-
rated ketones to unsaturated alcohols is demonstrated using water as a solvent. The XRD and FTIR spectra
show the highly ordered hexagonal nature of the MCM-41, even after encapsulation of the ruthenium
complex. The complex retains its configuration after anchoring, as was confirmed by FTIR and UV–Vis
analysis. The detailed reaction parametric effect was studied for the hydrogenation of 3-methylpent-3-
en-2-one to achieve complete conversion up to >99% chemoselectivity of 3-methylpent-3-en-2-ol. The
anchored heterogeneous catalysts were recycled effectively and reused five times with marginal changes
in activity and selectivity. The use of water as a solvent not only afforded high activity for the hydroge-
nation reaction compared to organic solvents, but also afforded a green process.
Keywords:
Ruthenium
1,10-Phenanthroline
Mesoporous materials
a,b-Unsaturated ketone
Chemoselective hydrogenation
Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved.
1. Introduction
supported on polymers [12] and MgO [13] have been reported as
heterogeneous catalysts for selective hydrogenation. Cinchonidine
a
,b-Unsaturated alcohols are of great commercial importance as
metal complexes have also been used for the hydrogenation of
unsaturated ketones to saturated ketones [14]. In all the above
reactions, relatively low to moderate chemoselectivity towards
unsaturated alcohols was obtained. Several attempts have been
made to develop suitable heterogeneous catalysts for the chemose-
they are widely used in fragrances, pharmaceutical industries,
intermediates in fine chemicals syntheses etc [1]. The chemoselec-
tive hydrogenation of a,b-unsaturated ketones to the correspond-
ing unsaturated alcohols using molecular hydrogen is quite
challenging because of the higher reactivity of the C@C bond com-
pared with that of the C@O group [2], i.e. the favorable thermody-
namics for hydrogenation of the C@C bond compared to the C@O
bond by ca. 35 kJ/mol [3]. Hence, the challenge is to achieve unsat-
urated alcohols via selective reduction of unsaturated ketones. In
addition, the product formed in the reaction may isomerize to
the corresponding saturated ketones, resulting in lower unsatu-
rated alcohol selectivity [4].
lective hydrogenation of the C@O group in
a,b-unsaturated alde-
hydes [15].
Ruthenium metal, owing to its 4d75s1 electron configuration,
has a wide range of possible oxidation states [16] and thereby
can stabilize various coordination geometries in each electronic
configuration. In particular, the lower oxidation states of ruthe-
nium complexes normally prefer trigonal-bipyramidal and octahe-
dral structures, such
a variety of structural geometries of
The reduction of unsaturated aldehydes or ketones has been re-
ported using various transition metal catalysts in the form of
homogeneous metal complexes, viz. diamine(ether-phosphine)
ruthenium complexes has great potential in selective hydrogena-
tion reactions.
The use of nitrogen containing metal complexes for hydrogena-
tion of unsaturated carbonyl compounds to unsaturated alcohols
has not been extensively studied to date. Also due to increasing
environmental concern in recent years for eco-friendly catalytic
systems, the use of water as a reaction medium has become an
important factor. The present study details with chemoselective
ruthenium(II) [5], g
6-p-cymene ruthenium(II) [6] and phenyldim-
ethylphosphine copper (I) complexes [7]. Supported metals like Ir
or Ru on H-b zeolite [8], immobilized Zr and Hf alkoxide on meso-
porous materials [9], gold supported on Fe2O3 [10,11] and gold
hydrogenation of a,b-unsaturated ketones with particular empha-
sis on 3-methylpent-3-en-2-one over homogeneous Ru–Phen-2 as
well as the heterogenized Ru–Phen-2-NH-MCM-41 catalyst in
aqueous medium.
⇑
Corresponding author at: Catalysis Division, National Chemical Laboratory,
Pune 411008, India. Tel.: +91 20 25902272; fax: +91 20 2590 2633.
E-mail address: amitdeshmukh18@gmail.com (A. Deshmukh).
0277-5387/$ - see front matter Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2010.09.006

A. Deshmukh et al. / Polyhedron 29 (2010) 3262–3268
3263
10 mL of acetonitrile and N,N-di-methyl-formamide (9:1 ratio)
were added as described earlier [19]. The mixture was then stirred
at room temperature for 12 h under a nitrogen atmosphere. The so-
lid product was filtered and washed with ethanol. Finally the solid
product was dried under vacuum at 80 °C. The grafted metal com-
plex in organo-functionalised NH-MCM-41 was confirmed by FTIR
and UV–Vis. The retained structure of MCM-41 was also confirmed
by XRD and TEM.
2. Material and methods
2.1. Synthesis and functionalization of MCM-41
The synthesis of the mesoporous material was carried out
hydrothermally as described earlier [17] in an autoclave under
autogenous pressure. In a typical synthesis procedure, 3 g fumed
silica (Aldrich) was added to a solution containing 0.64 g NaOH
(Merck) in 25 mL deionized H2O and stirred for 45 min. To this
mixture, a solution of 3.64 g CTABr (Loba Chemie) in 50 mL deion-
ized H2O was added dropwise under stirring, and the stirring was
continued for another 45 min. Finally, 37 mL deionized H2O was
added to the final synthesis gel, stirred further for 30 min and then
autoclaved at 100 °C for 36 h. The resultant molar gel composition
was 1 SiO2:0.32 NaOH:0.2 CTABr:125 H2O. The solid product thus
obtained after hydrothermal synthesis was filtered, washed thor-
oughly with distilled water and acetone, and dried at 70 °C under
vacuum for 12 h. After drying, the product was calcined at 540 °C
for 8 h. The solid product after calcination was characterized by
XRD for structure confirmation.
2.4. Catalytic hydrogenation reactions
The catalytic hydrogenation of 3-methylpent-3-en-2-one was
performed in a 100 mL high-pressure and high temperature auto-
clave at temperatures between 80 and 150 °C, at different H2 pres-
sures (1.37–2.75 MPa). In a typical reaction, 0.1 g of catalyst, 10%
(wt%) of potassium ter-butoxide (tBuOK) with respect to substrate
and 30 mL water as a solvent were used. The catalyst was recov-
ered by centrifugation and recycled for the same substrate under
identical reaction parameters.
The reaction mixtures were analyzed by an Agilent 6890 series
Before grafting the metal complex in MCM-41, the MCM-41 was
organo-functionalized by 3-aminopropyltrimethoxysilane (3APTS).
In the procedure, 1.0 g of calcined Si-MCM-41 was taken in 25 mL
dry toluene and stirred. To this stirred suspension, 0.60 g of diluted
APTS in 25 mL of dry toluene was added slowly under a N2 atmo-
sphere at room temperature. After complete addition of APTS, the
mixture was refluxed at 80 °C for 24 h in N2 atmosphere. The prod-
uct thus obtained was filtered, washed with dry toluene followed
by acetone and dried at 70 °C under vacuum. The functionalised
(NH-MCM-41) thus obtained was confirmed by elemental analysis.
gas chromatograph (GC) containing
a
capillary column
(30 m ꢀ 0.32 mm ꢀ 0.25
l
m film thickness) and flame ionization
detector. The products were also confirmed by GC–IR and GC–MS.
3. Results and discussion
3.1. Characterization of the catalyst
The powder XRD patterns recorded on a Rigaku-Mini-Flex
instrument of calcined Si-MCM-41 (Fig. 1 curve (a)), organofunc-
tionalized NH-MCM-41-P (post synthesis method) (Fig. 1 curve
(b)) and Ru–Phen-2-NH-MCM-41 (Fig. 1 curve (c)), are shown.
The XRD pattern of Si-MCM-41 shows four distinct characteristic
low angle reflections for the strong (1 0 0) reflection and feeble
(1 1 0), (2 0 0) and (2 1 0) reflections at 2h = 2.5°, 4.2°, 4.9° and
6.2°, respectively. The XRD patterns of each sample indicate a high
degree of order amongst the hexagonal mesophases of Si-MCM-41,
as reported earlier [20–22]. The XRD results show a decrease in the
peak intensity after functionalization and loading of the Ru com-
plex, which is due to the partial filling of mesopores by the func-
tional group and the Ru complex. This result was also supported
by a surface area and pore volume measurement study, given in
Table 1. Calcined MCM-41 shows the highest surface area
(1322 m2/g), which decreased by 15–20% and 22–28% by amine
surface modification and ruthenium phenanthroline complex load-
ing. The decrease in pore volume also confirms the anchoring of
the Ru metal complex in mesoporous MCM-41.
2.2. Synthesis of the metal complexes
The ruthenium metal complexes were prepared according to
the procedure available in literature [18]. In the complex synthesis,
an ethanolic solution of the phenanthroline ligand was added to an
aqueous solution of ruthenium under stirring conditions, and kept
overnight. The solid precipitate of the ruthenium phenanthroline
complex (abbreviated as Ru–Phen) was filtrated, washed with eth-
anol and dried at 70 °C for 12 h. Various metal complexes with dif-
ferent metal to ligand ratios (metal:ligand = 1–3) (Scheme 1) were
synthesized using the same methodology. Similarly ruthenium
bipyridine and pyridine complexes were also synthesized by the
same procedure with a metal to ligand ratio of 1:2, abbreviated
as Ru–Py and Ru–Bipy respectively. The complexes thus obtained
were characterized by UV–Vis and FTIR spectroscopy.
2.3. Grafting of metal complexes on to organo-functionalised MCM-41
The TEM images of Ru–Phen-NH-MCM-41 materials, shows a
clear hexagonal pattern of lattice fringes along the pore direction
(Fig. 2(A)). Parallel fringes due to the side-on view of the long pores
were also observed. The equidistant parallel fringes in the images
show the unique feature of separate layers and the addition of
the layers one after the other results in the formation of bunch of
layers.
The grafted ruthenium phenanthroline complexes (Scheme 2)
were obtained by taking 1 g of functionalized NH-MCM-41 in ace-
tonitrile. To this solution, 0.03 g of the predissolved complex in
+2
+2
+2
The selected area electron diffraction (SAED) pattern of the
sample (Ru–Phen-2-NH-MCM-41) is given in Fig. 2(B). The TEM
images and SAED patterns are well consistent with the regular hex-
agonal mesophases of MCM-41, with the homogeneity in the pat-
terns indicating the retention of the ordered patterns of MCM-41
after anchoring of the above mentioned Ru–Phen complexes. These
results corroborate the XRD results presented earlier (Fig. 1).
The effect of surface modification on the specific surface area,
pore volume and pore diameter values, estimated from N2 adsorp-
tion–desorption isotherms, is given in Table 1. Fig. 3 shows the N2
adsorption–desorption isotherm and pore size distribution curve
N
N
N
N
N
Ru
Ru
Ru
n
n
n
Where n= 1-3
Scheme 1. Schematic representation of the ruthenium metal complexes.

3264
A. Deshmukh et al. / Polyhedron 29 (2010) 3262–3268
(EtO)3Si(CH2)3NH2
N
1
Cl2
n
Ru
N
2
1
2
Ru(Phen)nCl
Si OH
Si O Si
(CH2)3NH2
Si O Si (CH2)3NH
Scheme 2. Immobilization of the Ru(Phen)nCl2 complex onto functionalised MCM-41.
a
b
c
2
4
6
8
10
2θ
Fig. 1. XRD pattern for (a) calcined MCM-41, (b) NH-MCM-41, (c) Ru–Phen-2-NH-
MCM-41, and (d) insitu-NH-MCM-41.
Table 1
Physical characteristics of various surface modified MCM-41 materials before and
after immobilization of metal complexes.
Material
Pore diameter (Å)
Pore volume
Surface area
(cm3 gꢁ1
)
(m2 gꢁ1
)
Si-MCM-41
NH2-MCM-41
Ru–Phen-1-NH-MCM-41
Ru–Phen-2-NH-MCM-41
Ru–Phen-3-NH-MCM-41
22.54
18.49
18.11
17.93
17.80
0.70
0.55
0.55
0.54
0.51
1322
1113
1104
1093
893
(inset). All the samples showed similar type IV isotherms, having
inflections around P/P0 = 0.35–0.8, characteristic of MCM-41 type
ordered mesoporous material. Calcined MCM-41 shows the highest
surface area, which decreases after surface modification for NH-
MCM-41 and Ru–(Phen)n-MCM-41 (n = 1–3), as expected. These
results indicate that partial filling of the mesopores occurred by
the aforesaid coordination of the ruthenium compounds, which
are anchored inside the pores of the mesoporous material.
FTIR spectra of all the neat ruthenium phenanthroline com-
plexes with different metal to ligand ratios are depicted in
Fig. 4(A). The peaks observed between wavenumbers 1000 and
1600 cmꢁ1 were attributed to bands for the framework-stretching
mode of the phenanthroline ligands. The absorption at ꢂ776 cmꢁ1
is characteristic of the cis confirmation of these complexes due to
the out of plane C–H deformation of the ligand [23].
Fig. 2. TEM images recorded from (A) Ru-NH-MCM-41 and (B) SAED patterns.
bridges cross-linking the silicate network [18]. In the case of NH-
MCM-41 with –(CH2)3NH2 (NH-MCM-41), the peak at 3305 and
3428 cmꢁ1 corresponds to the –NH2 group, whereas the transmis-
sion bands at 2935 and 2841 cmꢁ1, from the asymmetric and sym-
metric vibrations of the –CH2 group of the propyl chain of the
silylating agent, were indicative of successful anchoring of amine
moieties in the mesoporous material. The pore volume, average
pore diameters and BET surface area values of all the siliceous
and organo-functionalized MCM-41 samples are summarized in
Table 1. After grafting of the –NH2 functional group of the MCM-
41 materials, a decrease in surface areas, pore volumes and pore
diameters by ca. 15–20%, 6–11% and 22–28% respectively were ob-
served. All these results strongly indicate that the organic func-
tional groups are mainly located inside the channels.
Fig. 4(B) shows the FTIR spectra (a) NH-MCM-41 and (b) Ru–
Phen-2-NH-MCM-41 between 3200 and 400 cmꢁ1. The spectra
(Fig. 4(A and B)) obtained from template-free –NH-MCM-41 shows
characteristic bands at 1080, 796 and 452 cmꢁ1. Bands at similar
wavenumbers in the spectra of the crystalline and amorphous
SiO2 have been assigned to characteristic vibrations of Si–O–Si
The UV–Vis spectra of different metal to ligand ratios
[Ru(Phen)n]2+ complexes are shown in Fig. 5. The absorbance spec-
*
p–p phenan-
tra for these complexes in acetonitrile shows intense
throline intra-ligand transitions in the UV–Vis band at

A. Deshmukh et al. / Polyhedron 29 (2010) 3262–3268
3265
350
300
250
200
150
A
Volume adsorbed
volume desorbed
0.06
0.05
0.04
0.03
0.02
0.01
0.00
a
b
c
d
16 18 20 22 24 26 28 30
Diameter Å
200
300
400
500
600
0.2
0.4
0.6
0.8
1.0
Wavelength (nm)
P/P0
Fig. 3. N2 adsorption–desorption isotherms and corresponding pore size distribu-
tion curves (inset) for Si-MCM-41.
B
a
b
c
200
300
400
500
600
700
800
Wavenumber cm-1
Fig. 5. UV–Vis spectra of the (A) neat complex (a) Ru–Phen-1, (b) Ru–Phen-2, (c)
Ru–Phen-3, (d) Phen; and (B) immobilized metal complex (a) Ru–Phen-1-NH-MCM-
41, (b) Ru–Phen-2-NH-MCM-41, (c) Ru–Phen-3-NH-MCM-41.
complex in functionalised MCM-41 shows a characteristic band
at 510–530 nm, indicating successful anchoring of the complex
without any change in the configuration of the complex.
To find out the oxidation state of ruthenium before and after
immobilization, X-ray photoelectron spectroscopy (XPS) was per-
formed and the results are shown in Table 2. The Ru 3d5/2 and
Ru 3p3/2 BE values at ca. 280 and ca. 465 eV are evident for a RuII
species in the catalysts [26]. With increasing metal to ligand ratio,
the binding energy (Ru 3d5/2) of the complexes decreases, which is
attributed to the increase in the number of electrons in the outer-
most shell of the ruthenium metal, donated by the nitrogen atom
of the ligand. It is well known that as the number of electrons in
the outermost shell increases, the ionization energy decreases
(shielding effect), and as a result a decrease in the binding energy
was observed.
Fig. 4. FTIR spectra of (A) neat complex (a) Ru–Phen-1, (b) Ru–Phen-2, (c) Ru–Phen-
3; and (B) (a) NH-MCM-41, (b) Ru–Phen-2-NH-MCM-41.
Table 2
Core level binding energies (in eV) of various elements present in the catalyst
precursors and anchored catalysts.
500–530 nm and a second band of lesser intensity for metal-to-li-
gand charge-transfer (MLCT) transitions is between 340 and
390 nm [24]. The complexes also show a blue shift in the range
510–530 nm and which is characteristic of a cis configuration
[25]. The bathochromic shift for the cis-conformation is due to
the destabilization of the ruthenium t2g orbital because of electron
donation from the two anionic chloride ligands to the ruthenium
center. The UV–Vis patterns in the case of the immobilized metal
Material
Ru 3d5/2
Ru 3p3/2
N 1s
Si 2p
Ru–Phen-1
Ru–Phen-2
Ru–Phen-3
Ru–Phen-1-NH-MCM-41
Ru–Phen-2-NH-MCM-41
Ru–Phen-3-NH-MCM-41
280.4
280.3
279.8
280.5
280.3
279.8
465.1
465.0
464.8
465.2
465.1
465.0
401.5
401.6
401.5
401.5
401.5
401.6
–
–
–
103.4
103.4
103.5

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A. Deshmukh et al. / Polyhedron 29 (2010) 3262–3268
3.2. Catalytic hydrogenation reactions
3.2.1. Effect of metal to ligand ratio
Ru-Ligand-2
Ru-Ligand-2-NH-MCM-41
100
80
To understand the performance of the metal to ligand ratio, dif-
ferent complexes with ruthenium to phenanthroline ratios were
studied at 100 °C, 2.0 MPa and using water as a reaction medium,
and the results are shown in Fig. 6. RuCl3 as well as RuCl3–NH-
MCM-41 were also included in the studies for a comparative pur-
pose. Under homogeneous conditions, RuCl3 showed complete
conversion but lower selectivity compared to RuCl3 on MCM-41,
which clearly support the above conclusion that the accessibility
of the reactant molecule to metal is higher in RuCl3, while the con-
version remains comparable except for Ru–Phen-3. The selectivity
of the unsaturated alcohol goes on increasing in both cases (homo-
geneous and heterogeneous conditions) as the ruthenium to phe-
nanthroline ratio increases from one to three. The increase in the
selectivity for metal to ligand ratios of 2 and 3 (compared to 1) is
because of the steric effect of the ligand around active metal cen-
tre, but the rate of reaction decreases in the case of a metal to li-
gand ratio of 3, leading to a slower reaction over Ru–Phen-3 and
Ru–Phen-3-NH-MCM-41 catalyst. Therefore, further work was car-
ried out using Ru–Phen-2 and the Ru–Phen-2-NH-MCM-41 cata-
lyst as the best results were obtained with these catalysts. The
reaction of two equivalents of phenanthroline with RuCl3 provides
the octahedral [(phen)2RuCl2] complex containing cis-chlorides. In
this complex the two chloride ions can be displaced from the coor-
dination environment by reaction with another phenanthroline li-
gand to provide the cationic [(phen)3Ru]Cl2 complex [27]. Reaction
with one equivalent of phenanthroline, however, provides the
polymeric [(phen)RuCl2]n.
60
40
20
0
100
80
60
40
20
0
Py
Bipy
Ligand
Phen
Fig. 7. Effect of ligand on the conversion and selectivity of the hydrogenation of 3-
methylpent-3-en-2-one.
homogeneous catalysts. The same metal complexes, anchored in
NH-MCM-41 were used as heterogeneous catalysts. The reactions
were carried out at 100 °C and at 2.0 MPa using water as a reaction
medium and the results are given in Fig. 7.
The conversion exhibited by the Ru-complexes using all three
ligands was more or less comparable, ramping between 96% and
100%, whereas Ru–Phen-2 gave nearly quantitative conversion.
However, the selectivity for the unsaturated alcohol was lower
when the monodentate pyridine ligand was used, due to the for-
mation of 3-methylpentane-2-ol. This is due to more sites being
available for the reactant molecule to interact with the active me-
tal of the complex. Although the conversions using homogeneous
as well as heterogeneous catalyst systems were comparable, the
selectivity towards 3-methylpent-3-en-2-ol was slightly higher in
the case of the heterogeneous catalysts, in accordance with the
trend observed in the case of ketone hydrogenations.
For the catalytic transfer hydrogenation, an easily hydrolysable
ligand is necessary to initiate a Ru–H2 functionality [28]. Ru–Cl has
been recognized as an easily hydrolysable ligand in the literature.
Since the phenanthroline ligand in [(phen)3Ru]2+ is bound more
strongly than the Cl ligand, it follows that the reactivity of this
complex is lower than that of the [(phen)2RuCl2] complex. Simi-
larly, the bridging chloride ions in [(phen)RuCl2]n are more difficult
to hydrolyse, resulting in a lower rate.
3.2.3. Effect of solvent
3.2.2. Effect of different ligands
Table 3 depicts the effect of different solvents for the hydroge-
nation of 3-methylpent-3-en-2-one using Ru–Phen-2 and Ru–
Phen-2-NH-MCM-41 as catalysts. Except in n-butanol, almost com-
plete conversion was obtained in the other solvents (methanol and
iso-propanol) or reaction medium (water). It is quite interesting
that in spite of being biphasic (in the case of the homogeneous cat-
alytic system) or tri-phasic (in the case of the heterogeneous cata-
lytic system), the reaction was complete in the presence of water.
However, the selectivity for the primary product (unsaturated
The effect of ligand (pyridine as a monodentate ligand or 2,20-
bipyridyl and 1,10-phenanthroline as bidentate ligands) on the
activity and selectivity in the hydrogenation reaction of 3-methyl-
pent-3-en-2-one to 3-methylpent-3-en-2-ol was studied. The reac-
tions were carried out using Ru–Py-2, Ru–Phen-2 and Ru–Bipy-2 as
Ru-Phen
Ru-Phen-NH-MCM-41
100
80
60
Table 3
40
Effect of solvent on the selectivity and conversion of the hydrogenation of 3-
methylpent-3-en-2-onea.
20
S. No. Catalyst
Solvent
Conversion >C@O selectivity
0
(mole %)
(mole %)
100
1
2
3
4
5
6
7
8
Ru–Phen-2
Ru–Phen-2-NH-MCM-41
Ru–Phen-2
Ru–Phen-2-NH-MCM-41
Ru–Phen-2
Ru–Phen-2-NH-MCM-41
Ru–Phen-2
n-Butanol
81
80
100
100
69
75
79
87
86
92
95
99
80
60
40
20
0
Methanol
Iso-propanol 100
99
Water
100
99
Ru–Phen-2-NH-MCM-41
RuCl3
Ru-Phen-1
Metal to Ligand Ratio
Ru-Phen-2
Ru-Phen-3
a
Reaction conditions: substrate = 1 g, neat catalyst = 0.005 g (Ru–Phen-2), solid
catalyst = 0.1 g (Ru–Phen-2-NH-MCM-41), agitation = 300 rpm, substrate:base = 10
(wt/wt), temperature = 100 °C, pressure = 2.04 MPa, reaction medium = 30 mL
water.
Fig. 6. Effect of metal to ligand ratio on the conversion and selectivity of the
hydrogenation of 3-methylpent-3-en-2-one.

A. Deshmukh et al. / Polyhedron 29 (2010) 3262–3268
3267
alcohol) was significantly lower in the case of organic solvents vis-
à-vis water. In fact, the selectivity trend follows the order: n-buta-
nol < methanol < iso-propanol < water. This may be due to the high
cohesive energy density of water over other organic solvents used
during the reaction and the availability of hydrogen during the
reaction, as the solubility of hydrogen is less in water as compared
to other solvents [29].
3.2.4. Effect of temperature
Fig. 8 shows the effect of temperature on the activity and selec-
tivity of the catalytic hydrogenation of 3-methylpent-3-en-2-one
to 3-methylpent-3-en-2-ol. As the conversion temperature was in-
creased from 80 to 100 °C, the yield of the unsaturated alcohol in-
creased sharply from ca. 75% to 99%. However, a further increase in
temperature resulted in a decrease in selectivity towards the
unsaturated alcohol. This is quite expected, as at higher tempera-
tures, saturated alcohols will also be formed due to further
hydrogenation.
Fig. 9. Effect of pressure on the conversion and selectivity of the hydrogenation of
3-methylpent-3-en-2-one.
3.2.5. Effect of pressure
Fig. 9 depicts the effect of pressure on the conversion of 3-
methylpent-3-en-2-ol at 100 °C using water as a reaction medium
over Ru–Phen-2 catalyst. The hydrogen pressure shows a pro-
nounced effect on the conversion. There is a decrease in the selec-
tivity of 3-methylpentane-2-ol with an increase in hydrogen
pressure. At higher hydrogen pressures, further hydrogenation of
3-methylpent-3-en-2-ol to 3-methylpentane-2-ol may be possible
and as a result the selectivity decreases.
3.2.6. Effect of time
Fig. 10 shows a typical time profile for the hydrogenation of 3-
methylpent-3-en-2-one over Ru–Phen-2 and heterogenized Ru–
Phen-2-NH-MCM-41 catalysts at 100 °C, 2 Mpa pressure. In both
cases the conversion increases with reaction time, up to 4 h,
whereas the selectivity remains comparable throughout the reac-
tion. The reaction is faster in the case of the homogeneous catalyst
compared to the heterogeneous catalyst, as there is no diffusion
barrier/transport limitation for the reactant to adsorb on the sur-
face of the catalyst in the case of the homogeneous catalyst system.
The conversion after 4 h is almost 100%, however, the selectivity
for the heterogeneous catalyst (99%) was found to be slightly high-
er compared to the homogeneous catalyst (96%). A decrease in
Fig. 10. Effect of time on the conversion and selectivity of the hydrogenation of 3-
methylpent-3-en-2-one.
selectivity was observed when the reaction was prolonged, which
is due to the further hydrogenation of the unsaturated alcohol.
3.2.7. Catalyst recycling studies
The solid anchored catalyst was filtered and recycled three
times with a slight decrease in the activity and selectivity (Table
4). The decrease in conversion is due to the partial leaching of
ruthenium (ca. 11 ppm of ruthenium) from the total ruthenium
present in the solid catalyst. However, there is a very minor de-
Table 4
Recycling studies of the heterogeneous catalysts for the hydrogenation of 3-
methylpent-3-en-2onea.
No. of cycle
Conversion (mole %)
>C@O Selectivity (%)
Fresh catalyst
Recycle 1
Recycle 2
98
98
95
92
99
97
97
97
Recycle 3
a
Reaction conditions: substrate = 1 g, neat catalyst = 0.005 g (mole/mole) (Ru–
Phen-2), solid catalyst = 0.1 g (Ru–Phen-2-NH-MCM-41), agitation = 300 rpm, sub-
strate:base = 10 (wt/wt), temperature = 100 °C, pressure = 2.04 MPa, water = 30 mL,
time = 6 h.
Fig. 8. Effect of temperature on the conversion and selectivity of the hydrogenation
of 3-methylpent-3-en-2-one.

3268
A. Deshmukh et al. / Polyhedron 29 (2010) 3262–3268
Table 5
Effect of different substrates on the catalytic activity.a
O
OH
H2, Water, Catalyst
R1
R2
R1
R2
>C@O selectivity (%)
b
Catalysts
Substrate
Conversion (%)
TOF (hꢁ1
)
Ru–Phen-2
Ru–Phen-2-NH-MCM-41
Ru–Phen-2
Ru–Phen-2-NH-MCM-41
Ru–Phen-2
Ru–Phen-2-NH-MCM-41
Ru–Phen-2
R1 = C2H5, R2 = CH3
99
99
100
98
99
99
100
97
95
99
98
>99
96
98
94
97
246
212
241
201
257
247
363
339
R1 = C5H11, R2 = CH3
R1 = Ph, R2 = CH3
Cyclohex-2-ene-one
Ru–Phen-2-NH-MCM-41
a
Reaction conditions: substrate = 1 g, substrate: base = 10 (wt/wt), neat catalyst = 0.005 g (Ru–Phen-2), solid catalyst = 0.1 g (Ru–Phen-2-NH-MCM-41), tempera-
ture = 100 °C, pressure = 2.04 MPa, agitation speed = 300 rpm, and water = 30 mL.
b
TOF = turn over frequency = mole of product formed/mole of Ru per hour.
crease in selectivity using the recycled catalyst. The fresh reaction
was performed using the mother liquor and it was observed that
the conversion was around 9%. This takes the process towards a
green chemistry route in which a non-environmental pollutant sol-
vent and an easily separable and recyclable catalyst are used.
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4. Conclusion
The synthesis and catalytic activity of Ru(phen)2 immobilized
on MCM-41 has been successfully investigated. The rate of reaction
under homogeneous conditions is higher as compare to heteroge-
neous conditions, and the selectivity is lower. The heterogenization
of a homogeneous catalyst gives a higher selectivity for the product
compared to the reaction under homogeneous conditions for the
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Amit Deshmukh acknowledges the Council of Scientific and
Industrial research (CSIR), Govt. of India for research funding.