Heterogenized Ru(II) phenanthroline complex for chemoselective hydrogenation of diketones under biphasic aqueous medium

Article abstract of DOI:10.1016/j.molcata.2010.10.005

The chemoselective hydrogenation of acetylacetone to 4-hydroxypentan-2-one over immobilized ruthenium phenanthroline metal complexes in amino functionalized MCM-41 in biphasic aqueous reaction medium was investigated. The catalyst was characterized by XRD, TEM, surface analysis, FT-IR and UV-vis to understand the morphology, complex geometry, and XPS such that the oxidation state of the metal complex inside the MCM-41 framework could be understood. The use of water as a solvent, not only gives high activity and selectivity for hydrogenation of acetylacetone, but also gives a path for an environmentally safer process. The optimizations of ligand, metal to ligand ratio, the choice of solvent and other reaction parameters were studied in detail. The heterogeneous catalytic system gave a higher degree of chemoselectivity (99%) towards 4-hydroxypentan-2-one as compared to homogeneous catalyst when hydrogenation was carried out using water as a solvent. The immobilized ruthenium-phenanthroline complex was easily separated and reused.

Full text of DOI:10.1016/j.molcata.2010.10.005

Journal of Molecular Catalysis A: Chemical 333 (2010) 114–120
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Heterogenized Ru(II) phenanthroline complex for chemoselective hydrogenation
of diketones under biphasic aqueous medium
Amit Deshmukha,b,∗, Anil Kinagea, Rajiv Kumarc, Reinout Meijboomb
a Catalysis Division, National Chemical Laboratory, Pune 411008, India
b Chemistry Department, University of Johannesburg, Auckland Park, Johannesburg, South Africa
c Tata Chemicals Limited, 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 27 April 2010
Received in revised form 13 August 2010
Accepted 4 October 2010
Available online 1 November 2010
The chemoselective hydrogenation of acetylacetone to 4-hydroxypentan-2-one over immobilized ruthe-
nium phenanthroline metal complexes in amino functionalized MCM-41 in biphasic aqueous reaction
medium was investigated. The catalyst was characterized by XRD, TEM, surface analysis, FT-IR and UV–vis
to understand the morphology, complex geometry, and XPS such that the oxidation state of the metal
complex inside the MCM-41 framework could be understood. The use of water as a solvent, not only gives
high activity and selectivity for hydrogenation of acetylacetone, but also gives a path for an environmen-
tally safer process. The optimizations of ligand, metal to ligand ratio, the choice of solvent and other
reaction parameters were studied in detail. The heterogeneous catalytic system gave a higher degree of
chemoselectivity (99%) towards 4-hydroxypentan-2-one as compared to homogeneous catalyst when
hydrogenation was carried out using water as a solvent. The immobilized ruthenium–phenanthroline
complex was easily separated and reused.
Keywords:
Diketone
Acetylacetone
4-Hydroxypentan-2-one
Ruthenium
Phenanthroline
Chemoselective
Hydrogenation
Water
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
It also has great potential for exploitation of novel catalytic reac-
tions and variety of Ru complexes are prepared and used for various
Ketols are of immense importance for the synthesis of fine
chemicals [1], as they provide two different functional groups in
the same molecule that are ready for manipulation, one hydroxyl
and other carbonyl. For example, 4-hydroxypentan-2-one is used as
flavoring agent. Several attempts have been made to hydrogenate
diketones over cinchonidine [2–6], BINAP [7] and cyclooctadine
[8] metal complexes to diols. In all above studies significant for-
mation of the diol was obtained resulting in low chemoselectivity
towards ketol formation. Transition metal complexes of ruthenium,
rhodium, palladium and platinum containing nitrogen ligand viz.
phenanthroline and bipyridyl have also been utilized for the trans-
fer hydrogenation of carbonyl compounds [9]. Ruthenium metal is
getting more and more attention because of it is low cost, activity,
availability, etc. It is worth recalling that among the other transi-
tion metals Ru has 4d75s1 electronic configuration and hence it has
wide range of oxidation states from −2 to +8. Therefore, it forms
various coordination geometries in each electronic configuration.
reactions. In the case of bipyridyl or phenanthroline complexes,
the steric hindrance caused by the interaction between hydrogen
atoms of aromatic rings causes the Ru(II) complexes to posses the
cis geometrical conformation [10]. Therefore, it is interesting to
utilize these complexes for the hydrogenation of acetylacetone to
4-hydroxypentan-2-one. To the best of our knowledge this is the
first reported case in which a phenanthroline metal complex is used
for diketone hydrogenation.
The increased environmental concerns in recent years advocate
the replacement of organic solvents by water for environmentally
benign catalytic systems and also to use easily separable and recy-
clable catalyst in the reaction. The present study is extension of
our earlier work [11] which focused on the catalytic activity of
Ru–Phen-2 (number 2 indicates ruthenium: phenanthroline = 1:2)
and heterogenized Ru–Phen-2-NH–MCM-41 (Ru–Phen-2 encap-
sulated in amino functionalized MCM-41) for chemoselective
hydrogenation of acetylacetone to 4-hydroxypentan-2-one using
water as the preferred reaction medium. Initially the system behav-
ior is triphasic, but after completion, the reaction behavior is
biphasic due to the fact that the ketol formed during the reaction is
soluble in water. Some other biphasic systems and different dike-
tone hydrogenation results are also reported for the comparison.
∗
Corresponding author at: Catalysis Division, National Chemical Laboratory, Pune
411008, India. Tel.: +91 20 25902272; fax: +91 20 25902633.
E-mail address: amitdeshmukh18@gmail.com (A. Deshmukh).
1381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.10.005

A. Deshmukh et al. / Journal of Molecular Catalysis A: Chemical 333 (2010) 114–120
115
+2
+2
+2
(EtO)3Si(CH2)3NH2
N
N
1
Cl2
Ru
N
N
N
N
N
Ru
Ru
n
Ru
2
1
2
Si OH
Si O Si (CH) NH2
Ru(Phen)nCl
n= 1, 2, 3
SiO Si (CH2)3NH
2 3
n
n
n
Where n= 1-3
Scheme 2.
Scheme 1. Schematic representation of ruthenium metal complexes.
2.4. Catalytic hydrogenation reactions
2. Experimental
In a typical reaction, 0.1 g of catalyst in the case of heterogenized
complex system is used. In homogeneous complex system, the sub-
strate to metal ratio was 1000. In both the cases, 10% of potassium
ter-butoxide (tBuOK) with respect to the substrate as a base and
30 mL water as a solvent was placed in a 100 mL high pressure auto-
clave. The catalytic hydrogenation of acetylacetone was performed
at different temperatures (100–150 ◦C), at different H2 pressures
(1.37–2.75 MPa) and at stirring at 300 rpm, using water as a sol-
vent. The reaction mixtures were analyzed on an Agilent 6890 series
gas chromatograph (GC) containing 10% ␤-cyclodextrin capillary
column (30 m × 0.32 mm × 0.25 ␮m film thickness) and flame ion-
ization detector. The products were also confirmed by GC–MS. After
reaction the catalyst was separated by centrifugation and recycled
for the same substrate under identical reaction parameters.
2.1. Synthesis of metal complexes
Ruthenium metal complexes were prepared according to the
procedure available in literature [12]. In a typical complex synthe-
sis, ethanolic solution of the phenanthroline ligand was added to
the aqueous solution of ruthenium under stirring conditions and
kept overnight. The solid precipitate was filtered, washed with
ethanol, and dried at 70 ◦C for 12 h. Similarly, ruthenium bipyri-
dine and pyridine complexes were also synthesized by the same
procedure with a metal to ligand ratio of 1:2. The complexes thus
obtained were characterized by UV–vis and FT-IR spectroscopy.
2.2. Synthesis of MCM-41 mesoporous material
3. Results and discussion
The synthesis of
a mesoporous material was carried out
3.1. Characterization of catalyst
hydrothermally as described earlier [13] in an autoclave under
autogeneous pressure. 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 then dried at 70 ◦C
temperature under vacuum for 12 h. After drying the product was
calcined at 540 ◦C for 8 h. The structure of MCM-41 was con-
firmed by XRD. The MCM-41 was then amino-functionalized by
3-aminopropyltrimethoxy-silane (APTS). In this procedure, 0.60 g
of diluted APTS in 25 mL of dry toluene was added slowly under an
3.1.1. Powder X-ray diffraction
The XRD (powder) pattern (Fig. 1) of calcined Si–MCM-41 (curve
(a)), organofunctionalized NH–MCM-41-P (post synthesis method)
(curve (b)), Ru–Phen-2-NH–MCM-41 (curve (c)) are shown. The
XRD pattern of Si–MCM-41 shows four characteristic low angle
reflections [strong 100 reflection and weak (1 1 0), (2 0 0) and (2 1 0)
reflections] at 2ꢀ = 2.1◦, 3.7◦, 4.3◦ and 5.6◦ respectively. The XRD
patterns indicate high degree of order-ness amongst the hexagonal
(p6mm) mesophases of Si–MCM-41 [15–17]. The results indicate an
ordered mesoporous material even after immobilization of metal
complex. However, a slight decrease in the peak intensities were
observed in the case of Ru-complexes containing samples, which
N2 atmosphere at room temperature to a suspension of 1.0 g of cal-
cined Si–MCM-41 in 25 mL dry toluene. After complete addition of
APTS, the mixture was refluxed at 80 ◦C for 12 h in N2 atmosphere.
The product thus obtained was filtered, washed with dry toluene
followed by acetone and then dried at 70 ◦C under vacuum. The
amino functionalized (NH–MCM-41) material thus obtained was
confirmed by elemental analysis, FT-IR, UV–vis spectroscopy.
2.3. Grafting of metal complexes on to amino-functionalized
MCM-41
The grafted ruthenium phenanthroline complex (Scheme 1) was
obtained by first taking 1 g NH–MCM-41 in acetonitrile. To this
solution 0.03 g of the pre-dissolved complex in 10 mL of acetonitrile
and N,N-di-methyl-formamide (9:1 ratio) was added as described
earlier [14]. The mixture was then stirred at room temperature for
12 h under a nitrogen atmosphere (Scheme 2). The solid product
was filtered and washed with ethanol. Finally the solid product
was dried under vacuum at 80 ◦C. The grafted metal complex in
amino-functionalized MCM-41 was confirmed by FT-IR and UV–vis
spectroscopy. The retained structure of MCM-41 was also con-
firmed by XRD.
Fig. 1. XRD pattern for (a) calcined Si–MCM-41, (b) NH–MCM-41 and (c) Ru–Phen-
2-NH–MCM-41.

116
A. Deshmukh et al. / Journal of Molecular Catalysis A: Chemical 333 (2010) 114–120
Table 1
Physical characteristics of various surface modified MCM-41 before and after immobilization of metal complexes.
Material
Pore diameter (Å)
Pore volume (cm3 g−1
)
Surface area (m2 g−1
)
Si–MCM-41
NH–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
350
Ru-Phen-NH-MCM-41
300
250
200
150
400
350
300
250
200
150
Si-MCM-41
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
P/P0
Fig. 2. N2 adsorption–desorption isotherms and corresponding pore size distribu-
tion curves (inset) for Si–MCM-41 and Ru–Phen–NH–MCM-41.
might be due to the partial filling of Ru-complexes inside the meso-
pores. The d spacing and unit cell parameters of the samples are
given in Table 1.
3.1.2. Specific surface area
Table 1 depicts the effect of surface modification on the specific
surface area, pore volume and pore diameter values as estimated
from N2 adsorption–desorption isotherms. Fig. 2 shows the N2
adsorption–desorption isotherm and pore size distribution curve
(inset). All the samples showed a similar type IV isotherm having
inflection around P/P0 = 0.35–0.8 which is characteristic of MCM-
41 type ordered mesoporous materials. Calcined MCM-41 showed
highest surface area, which goes on decreasing after surface mod-
ification for NH–MCM-41 and Ru–(Phen)n–MCM-41 (n = 1, 2, 3), as
expected. These results indicate that partial filling of mesopores
occurred by the aforesaid coordination of compounds, which are
anchored inside the pores of 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 observed. All these results strongly indi-
cate that the organic functional groups are mainly located inside
the channels of the support.
Fig. 3. FT-IR spectra of the (A) neat complex (a) Ru–Phen-1 and (b) Ru–Phen-2, (c)
Ru–Phen-3; and (B) (a) NH–MCM-41 (b) Ru–Phen-2-NH–MCM-41.
Fig. 3(B) shows the FT-IR spectra (a) NH–MCM-41 and (b)
Ru–Phen-2-NH–MCM-41 between 3200 and 400 cm−1. Spectrum
(Fig. 3(A) and (B)) obtained from template-free –NH–MCM-41
shows characteristic bands at 1080, 796 and 452 cm−1. Bands at
similar wavenumber in the spectra of the crystalline and amor-
phous SiO2 have been assigned to characteristic vibrations of
Si–O–Si bridges cross-linking the silicate network [19]. In the case
of NH–MCM-41 with –(CH2)3NH2 (NH–MCM-41), the peak at 3305
and 3428 cm−1 corresponds to the –NH2 group, whereas the trans-
mission bands at 2935 and 2841 cm−1 from the asymmetric and
symmetric vibrations of –CH2 group of the propyl chain of the
silylating agent were indicative of successful anchoring of amine
moieties in the mesoporous material.
3.1.3. FT-IR spectra
FT-IR spectra of all the neat ruthenium–phenanthroline com-
plexes with different metal to ligand ratios are depicted in Fig. 3(A).
The peaks observed between wavenumber 1000 and 1600 cm−1
were attributed to bands for the framework-stretching mode of
the phenanthroline ligand. The band at ∼776 cm−1 (Fig. 3(A) inset)
characteristic of the Cis confirmation of complex which is due to
the out-of-plane C–H deformation of the ligand [18].
3.1.4. UV–vis spectra
The UV–vis spectra of six-coordinate [Ru(phen)2]2+ complexes
are shown in Fig. 4. The absorbance spectra for these com-
plexes in acetonitrile are characterized by the presence of intense
ꢁ–ꢁ* phenanthroline intraligand transitions in the UV–vis region

A. Deshmukh et al. / Journal of Molecular Catalysis A: Chemical 333 (2010) 114–120
117
Table 2
Core level binding energies (in eV) of various elements present in the catalyst pre-
cursors and anchored catalysts.
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
ization energy decreases (shielding effect) and as a result the bond
strength between the metal and the ligand decreases, which in turn
decreases the binding energy.
3.1.6. Transmission electron microscopy
Fig. 5(A) represents the TEM images of Ru–Phen–NH–MCM-41
materials, showing a clear hexagonal pattern of lattice fringes along
the pore direction. 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 addi-
tion of the layers one after other results in the formation of a bunch
of layers.
The selected area electron diffraction (SAED) pattern of the sam-
ple (Ru–Phen-2-NH–MCM-41) is given in Fig. 5(B). The TEM images
Fig. 4. UV–vis spectra of the (A) neat complex (a) Ru–Phen-1, (b) Ru–Phen-2, (c)
Ru–Phen-3 and (d) Phen; and (B) immobilized metal complex (a) Ru–Phen–1-
NH–MCM-41, (b) Ru–Phen-2-NH–MCM-41 and (c) Ru–Phen-3-NH–MCM-41.
and metal-to-ligand charge-transfer (MLCT) bands at 500–530 nm
along, with a second band of lesser intensity between 340 and
390 nm [20]. Interestingly the complexes show a blue shift in the
range of 510–530 nm which is characteristic of the Cis configura-
tion and is attributed to the electron donating chlorine group [21].
The bathochromic shift in cis-conformation is due to the destabi-
lization of the ruthenium t2g orbital because of electron donation
from the two anionic chloride ligands to ruthenium center.
The UV–vis patterns observed in the case of immobilized metal
complexes on surface modified MCM-41 are shown in Fig. 4(B).
Curves a–c represents Ru–Phen-1-NH–MCM-41, Ru–Phen-2-
NH–MCM-41, Ru–Phen-3-NH–MCM-41 samples respectively. The
band at 510–530 nm clearly indicates the anchoring of the complex
in the surface modified MCM-41 without change in the configura-
tion of the complex.
3.1.5. X-ray photoelectron spectra
X-ray photoelectron spectroscopy (XPS) was used to find out the
oxidation state of ruthenium in the neat and immobilized complex
before and after reactions. Table 2 represents the Ru 3d5/2, Ru 3p3/2
and N 1s core level binding energy (BE) obtained from XPS anal-
ysis. BE values for Ru 3d5/2 and Ru 3p3/2 were ca. 280 and 465 eV
respectively, making it evident that the ruthenium in the catalysts
was present as RuII species [22]. The binding energy of Ru 3d5/2 in
complexes goes on decreasing from top to bottom through Table 2,
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 from the ligand (as metal to ligand ratio goes on increasing).
As the number of electrons in outer most shell increases, the ion-
Fig. 5. TEM images recorded from (A) Ru–NH–MCM-41 and (B) SAED patterns.

118
A. Deshmukh et al. / Journal of Molecular Catalysis A: Chemical 333 (2010) 114–120
Fig. 7. Effect of different ligand on conversion and selectivity in hydrogenation
of acetylacetone to 4-hydroxypentan-2-one over Ru–Phen-2 and Ru–Phen-2-
NH–MCM-41.
Fig. 6. Effect of metal to ligand ratio on conversion and selectivity in hydrogenation
of acetylacetone to 4-hydroxypentan-2-one over Ru–Phen-2 and Ru–Phen-2-
NH–MCM-41.
pentan-2-one was slightly higher in case of the heterogeneous
catalyst.
and SAED patterns are well consistent with the regular hexagonal
mesophases of MCM-41, with homogeneity in patterns indicating
the retention of the ordered patterns of MCM-41 after anchoring
of the abovementioned Ru–Phen complexes. These results corrob-
orate the XRD results presented earlier (Fig. 1).
3.2.3. Effect of solvent
To find out the suitable reaction medium for the hydrogenation
of acetylacetone, the reactions were carried out using Ru–Phen-
2 and Ru–Phen-2-NH–MCM-41 catalysts with different solvents
(Table 3). Both, homogeneous and heterogeneous conditions, the
conversions are found comparable, however, the selectivity of 4-
hydroxypentan-2-one was lower in case of homogeneous condition
as compared to heterogeneous condition. The uses of water as a
solvent gave higher conversion and selectivity compared to dif-
ferent alcohols, as the solubility of hydrogen is higher in alcohol
as compared to water therefore reaction tended towards total
hydrogenation. Therefore, for further studies water, a universal and
environmentally benign solvent was used. In aqueous solution the
Ru–(Phen)2Cl2 complex shows metal-to-ligand charge transfer in
the exited state. The increased electron density in the phenanthro-
line ligand system has the effect of turning the excited state of
nitrogen in the ligand into a stronger basic group. In water, this
group abstract a proton from dissolved H2 in solvent to form the
protonated ground-state species, relaxing to the starting mate-
rial in the process. In protonic solvents this protonated pathway
is precluded and the charge-transfer state is in equilibrium with
the excited metal species that then collapses to the ground state
with emission of proton [23–25]. This type of activation of molec-
ular splitting of hydrogen by Ru–(Phen)2Cl2 complex in aqueous
media leads the higher rate of hydrogenation reactions in aqueous
medium than in alcoholic medium
3.2. Catalytic hydrogenation reactions
3.2.1. Effect of metal to ligand ratio
The effect of metal to ligand ratio was investigated to achieve the
maximum selectivity which is shown in Fig. 6. Different complexes
with different ruthenium to phenanthroline ratios were studied at
the 120 ◦C, 2.0 MPa and using water as a solvent. Although, com-
parable conversions were obtained in the case of homogeneous as
well as heterogeneous conditions, the heterogenized metal com-
plex showed an increase in selectivity, which is due to the diffusion
barrier for the reactant. The selectivity of 4-hydroxypentan-2-one,
in both homogeneous and heterogeneous conditions, increased as
the ruthenium to phenanthroline ratio decreased from one to three.
This may be attributed the steric effect of ligand molecule, which
controls the approach of the reactant molecule towards the reactive
metal center in complex.
3.2.2. Effect of different ligand
The effect of ligand (pyridine as a mono or 2,2ꢀ-bipyridyl and
1,10-phenanthroline as a bi-dentate) on the activity and selectiv-
ity of the catalyst in the hydrogenation reaction of acetylacetone
to 4-hydroxypentan-2-one is shown in Fig. 7. The reactions were
carried out using Ru–Py-2, Ru–Phen-2 and Ru–Bipy-2 as a homoge-
neous catalysts. The same metal complexes were also anchored in
NH–MCM-41 and were used as heterogeneous catalyst. The reac-
tions were carried out at 120 ◦C and at 2.0 MPa using water as a
reaction medium and the results are given in Fig. 7.
Table 3
Effect of different solvents on conversion and selectivity for hydrogenation of
acetylacetonea.
Sl.no.
Catalyst
Solvent
% Conv.
Ketol Sel. (%)
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
85
80
90
88
100
98
75
79
82
85
87
92
99
99
The conversion exhibited by Ru-complexes using all the
three ligands is comparable. However, there was significant loss
of selectivity towards alcohol (due to the formation of 2,4-
dihydroxypentane), when mono-dentate ligand pyridine was used.
The cause of this is may be the less steric hindrance around the
active metal center (as compared to bi-dentate ligand), which facil-
itates ketol to interact and further hydrogenate to diol. Although,
the conversions using homogeneous as well as heterogeneous
catalyst systems were comparable, the selectivity for 4-hydroxy-
Methanol
Iso-propanol
Water
100
100
Ru–Phen-2-NH–MCM-41
aReaction
conditions:
substrate = 1 g,
substrate:base = 10
(w/w),
substrate:catalysis = 1000 (mole/mole) (Ru–Phen-2), in case of Ru–Phen-2-
NH–MCM-41 = 0.1 g, temp. = 120 ◦C, pressure = 2.04 MPa, agitation speed = 300 rpm,
30 mL water.

A. Deshmukh et al. / Journal of Molecular Catalysis A: Chemical 333 (2010) 114–120
119
Table 4
Recycle studies of the heterogeneous catalysts for hydrogenation of acetylacetone.
Entry
Catalyst
% Conv.
Ketol Sel. (%)
TONa
1
2
3
4
Ru–Phen-2-NH–MCM-41
98
97
98
89
99
98
95
95
1921
1902
1745
1504
Ru–Phen-2-NH–MCM-41-R1
Ru–Phen-2-NH–MCM-41-R2
Ru–Phen-2-NH–MCM-41-R3
a
Reaction conditions: substrate = 1 g, substrate:base = 10 (w/w), 0.05 g (Ru–Phen-
2), in case of Ru–Phen-2-NH–MCM-41 = 0.1 g, temp. = 120 ◦C, pressure = 2.04 MPa,
agitation speed = 300 rpm, 30 mL water.
55
50
45
40
35
30
25
20
15
Fig. 8. Effect of temperature on conversion and selectivity in hydrogenation
of acetylacetone to 4-hydroxypentan-2-one over Ru–Phen-2 and Ru–Phen-2-
NH–MCM-41.
3.2.4. Effect of temperature
To find out the optimum reaction temperature for the hydro-
genation of acetylacetone, reactions were carried out at various
temperatures and the results are shown in Fig. 8. It was observed
in both the homogeneous as well as heterogeneous cases that,
with increase in temperature from 100 to 150 ◦C the conver-
sion increases and selectivity of 4-hydroxypentan-2-one decreases.
The highest selectivity (99%) was observed at 120 ◦C reaction
temperature. However, the selectivity decreases from 120 ◦C
onwards due to further hydrogenation of acetylacetone to form
2,4-pentane-diol. In heterogeneous condition the selectivity of 4-
hydroxypentan-2-one is higher compared to that of homogeneous
condition at all the temperatures.
500
750
1000
1250
1500
1750
2000
Substrate : Metal
Fig. 10. Effect of catalyst loading on conversion of acetylacetone.
was obtained at higher pressure of hydrogen in both homoge-
neous and heterogeneous reaction conditions. There is a decrease in
the selectivity of 4-hydroxypentan-2-one as H2 pressure increases
from 1.36 to 2.72 MPa as more hydrogen is available to further
hydrogenate 4-hydroxypentan-2-one to diol. The heterogeneous
catalyst shows higher chemoselectivity than the homogeneous cat-
alyst although the conversion is higher for homogeneous catalyst.
3.2.5. Effect of pressure
Fig. 9 depicts the effect of pressure on chemoselectivity of
4-hydroxypentan-2-one at 120 ◦C using water as a solvent. The
hydrogen pressure shows a pronounced effect on the conversion
and selectivity. As expected, the high conversion and low selectivity
3.2.6. Recycle studies
Table 4 depicts the recyclability of the heterogenized cata-
lyst. The heterogenized catalyst can be recycled three times. The
selectivity and conversion decreases to 95% and 89%, respectively,
at the time of third recycle. The decrease in the conversion and
selectivity is attributed to the partial leaching of ruthenium (0.9%
of total ruthenium loaded) from the catalyst and break down of
metal complex respectively. A fresh reaction was performed using
mother liquor of previous reaction and around 10% conversion was
observed which is due to the partial leaching of ruthenium.
3.2.7. Effect of agitation
The effect of agitation speed was performed (200–1000 rpm) to
ascertain the liquid–solid mass transfer effect. The data obtained
showed that there is o effect of agitation on the selectivity pattern,
indicating that the reaction is not dependent on liquid–solid mass
transfer barrier.
3.2.8. Effect of catalyst loading
Effect of catalyst loading on the conversion of acetylacetone was
investigated for substrate to metal ratio in the range of 500–2000
and the results are shown in Fig. 10. The conversion was observed
to increase linear dependent on the amount of catalyst loading,
indicating the absence of external liquid–solid mass transfer.
Fig. 9. Effect of pressure on conversion and selectivity in hydrogenation of acety-
lacetone to 4-hydroxypentan-2-one over Ru–Phen-2 and Ru–Phen-2-NH–MCM-41.

120
A. Deshmukh et al. / Journal of Molecular Catalysis A: Chemical 333 (2010) 114–120
4. Conclusion
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In general, the rate of the reaction in homogeneous condition is
higher as compared to heterogeneous condition and the selectiv-
ity is lower. The heterogenization of homogeneous catalyst gives
higher selectivity (99%) of the product compared to the reaction
in homogeneous condition (%96) at the same conversion. The cis
geometry of the metal complex was retained even after immobi-
lization of metal complex in MCM-41. The heterogeneous catalyst
can be recycled for three times with slight decrease in the activ-
ity which is due to the leaching of 0.9% of ruthenium with respect
to the total amount of ruthenium loaded. Bi-dented ligands show
higher activity and selectivity of the product in hydrogenation
of acetylacetone to 4-hydroxypentan-2-one because of reducing
the accessibility of reactant molecule towards the metal center,
thus lowering the rate of the reaction. The use of water gives
another benefit of green chemistry and higher selectivity to 4-
hydroxypentan-2-one for the hydrogenation of acetylacetone.
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Acknowledgement
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Amit Deshmukh acknowledges Council of Scientific and Indus-
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