Selective Hydrogenation of Diketones on Supported Transition Metal Catalysts

Article abstract of DOI:10.1007/s10562-019-02983-5

Abstract: The hydrogenation of α-diketones yields α-hydroxyketones or vic-diols, both compounds of great interest in fine chemistry. The reaction tests were the liquid phase hydrogenation of 2,3-butanedione and 2,3-pentanedione at mild conditions. The objectives of this work were evaluating the effect over the activity and selectivity of: (a) different transition metallic phase based catalysts supported on activated carbon, (b) the symmetry of the reactants and (c) solvents. The physicochemical characterization of the catalysts was carried out by ICP, XRD, TEM, N₂ adsorption and XPS. The keto-enol equilibrium of diketones was studied by 1H-NMR. All the catalysts were active in both reactions. In terms of activity, Pt and Rh were the best active phases. For both reactants the highest selectivity towards hydroxyketones were achieved with Pd, while Ru was the most selective towards the diol. Both the activity and selectivity followed similar patterns in the hydrogenation of both diketones. The greater activity of Pt was attributed to the high dispersion of the active metal phase in this catalyst and the high efficiency of Pt for C = O bond reduction. The high selectivity of the Pd catalysts towards the intermediate product was attributed to many effects: (i) a lower interaction of the hydroxyketone with the active site as compared to the diketone, (ii) the easy reducibility of the C = C double bond on Pd, provided by the keto-enol tautomerism of diketones.

Full text of DOI:10.1007/s10562-019-02983-5

Catalysis Letters
https://doi.org/10.1007/s10562-019-02983-5
Selective Hydrogenation of Diketones on Supported Transition Metal
Catalysts
N. Carrara1 · J. M. Badano1 · S. Vailard2,3 · C. Vera1,2 · M. Quiroga1,2
Received: 29 May 2019 / Accepted: 29 September 2019
© Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
The hydrogenation of α-diketones yields α-hydroxyketones or vic-diols, both compounds of great interest in fne chemistry.
The reaction tests were the liquid phase hydrogenation of 2,3-butanedione and 2,3-pentanedione at mild conditions. The
objectives of this work were evaluating the efect over the activity and selectivity of: (a) diferent transition metallic phase
based catalysts supported on activated carbon, (b) the symmetry of the reactants and (c) solvents. The physicochemical
characterization of the catalysts was carried out by ICP, XRD, TEM, N2 adsorption and XPS. The keto-enol equilibrium of
diketones was studied by 1H-NMR. All the catalysts were active in both reactions. In terms of activity, Pt and Rh were the
best active phases. For both reactants the highest selectivity towards hydroxyketones were achieved with Pd, while Ru was
the most selective towards the diol. Both the activity and selectivity followed similar patterns in the hydrogenation of both
diketones. The greater activity of Pt was attributed to the high dispersion of the active metal phase in this catalyst and the
high efciency of Pt for C=O bond reduction. The high selectivity of the Pd catalysts towards the intermediate product was
attributed to many efects: (i) a lower interaction of the hydroxyketone with the active site as compared to the diketone, (ii)
the easy reducibility of the C=C double bond on Pd, provided by the keto-enol tautomerism of diketones.
Graphic Abstract
Keywords Selective hydrogenation · α-Diketones · Transition metals · Palladium · Hydroxyketones
1
* J. M. Badano
Instituto de Investigaciones en Catálisis y Petroquímica
(INCAPE-CONICET), Ruta Nac. 168 km 0—Paraje El Pozo,
3000 Santa Fe, Argentina
jbadano@fq.unl.edu.ar
N. Carrara
2
3
ncarrara@fq.unl.edu.ar
Facultad de Ingeniería Química (UNL, FIQ), Universidad
Nacional del Litoral, Santa Fe, Argentina
S. Vailard
svaillard@intec.unl.edu.ar
Instituto de Desarrollo Tecnológico para la Industria Química
(INTEC-CONICET), Santa Fe, Argentina
C. Vera
cvera@fq.unl.edu.ar
M. Quiroga
mquiroga@fq.unl.edu.ar
Vol.:(0123456789)
1 3

N. Carrara et al.
For the reaction sequence A→B→C it can be deduced
that in order to produce B with high selectivity the following
aspects must be considered:
1 Introduction
The hydrogenation of α-diketones is a reaction of great inter-
est in the industries of fne chemicals and chemical special-
ties. The products are α-hydroxyketones and/or vic-diols as
shown in Fig. 1. The α-hydroxyketones (e.g. acetoin, acetyl
ethyl carbinol, propionyl methyl carbinol, etc.) are usually
employed as chemical intermediates or as active components
in the industry or scents and fragrances [1, 2]. Vic-Diols
(e.g. 2,3-butanediol, 2,3-pentanediol, 2,3-hexanediol, etc.)
are usually employed as solvents and humidifers in the cos-
metics industry, and as plasticizers or intermediates for the
polymers industry (Vulkollan) [3]. Though currently being
of great interest, only a few reports on the simple hydrogena-
tion of α-diketones [4–6] or their chiral hydrogenation [7–9]
can be found in the open literature.
– The reaction should be performed under chemical con-
trol in order to the intrapellet difusion resistance. If the
residence time inside the pellet is reduced, desorption
of the intermediate B product is favored and so, total
hydrogenation to C is disfavored.
– Mild reaction conditions are preferred so that the energy
barrier for deep hydrogenation is not surpassed and C
formation is disfavored. High temperatures (T>398 K)
and pressures (P>100 bar) favor the reactions of overhy-
drogenation and hydrogenolysis, thus reducing the yields
to the desired products [15]. Some side reactions are also
responsible for catalyst deactivation.
– Low hydrogen concentrations are preferred in order to
disfavor the consecutive reaction intermediates leading
to deep saturation.
Duan et al. [4] hydrogenated butanedione and acetoin
in the gas phase, obtaining selectivity of 99.9% towards
2,3-butanediol when using Ni/SiO2 catalysts. Zuo et al. [10]
studied the hydrogenation of 2,3-butanedione in the liquid
phase employing Pt/Al2O3 catalysts modifed with diferent
transition metal promoters.
Carbonaceous catalysts are widely used in industrial
reactions, especially for hydrogenation processes [16]. In a
recent review Campisciano et al. [12] focused the attention
on the carbon nanoforms in metal-based reactions, among
them hydrogenation reactions of ketones. These catalysts
have several advantages in comparison to alumina and silica
supported catalysts, e.g. low cost, inertness, high superfcial
area and stability. Also these catalysts show low deactiva-
tion; it is possible to modify the chemical surface during
the catalyst preparation and it is easy to recover the metallic
phase from the spent catalysts [16].
The platinum VIII group (Ru, Rh, Pd, Os, Ir and Pt)
have been used as hydrogenation catalysts. During catalytic
hydrogenation, the second-row group VIII metals (Ru, Rh,
Pd) often show behavior diferent from that of the third-row
group (Os, Ir, Pt), e.g. the former give substantial isomeriza-
tion in olefn hydrogenation, while the second group no [11].
Besides Ru and Rh are highly active and selective catalysts,
particularly, for the hydrogenation of aromatic compounds
[12]. On the other hand, Os and Ir have found much less use
as hydrogenation catalysts, although their high selectivity
but very low activity in a few processes as the hydrogenation
of α,β-unsaturated aldehydes to allylic alcohols [11] or the
hydrogenation of crotonaldehyde [13], besides, at mild con-
ditions Ir is a well-known hydrogenolysis catalyst of carbon
carbon bonds [14].
So the objectives of this work were evaluating the efect
over the activity and selectivity during the hydrogenation
of α-diketones of: (a) diferent commercial and synthe-
sized transition metallic phase based catalysts supported on
activated carbon, (b) the symmetry of the reactants and (c)
several solvents. The reaction tests were the liquid phase
hydrogenation of symmetrical 2,3-butanedione (BD) and
non-symmetrical 2,3-pentanedione (PD). In the hydro-
genation of BD, the main products are acetoin (AC) and/
or 2,3-butanediol (BDO). In the hydrogenation of PD the
products are 2-hydroxy-3-pentanone (2H3P), 3-hydroxy-
2-pentanone (3H2P) and/or 2,3-pentanediol (PDO).
2 Experimental
2.1 Catalysts Preparation
The method of incipient wetness impregnation was used
for preparing the Pt catalyst. The chosen activated carbon
was Norit RX3. This was impregnated with an acidic solu-
tion of the H2PtCl6 precursor at pH=1 with HCl. After the
Fig. 1 Scheme of consecutive hydrogenation of a α-diketone to b
α-hydroxyketone and c vic-diol
1 3

Selective Hydrogenation of Diketones on Supported Transition Metal Catalysts
impregnation with the acidic solution, the catalyst was dried
overnight at 373 K. The theoretical mass percentage of Pt in
the fnal catalyst was 5% (5Pt-C).
2.3 Catalytic Activity Tests
The catalysts were tested with the hydrogenation reac-
tions of 2,3-butanedione (BD) and 2,3-pentanedione (PD).
The catalytic tests were performed in a stirred tank reactor
(100 mL Parr type) with PTFE-coated internals. The used
solvents were isopropanol (Cicarelli, 99.5%) and cyclohex-
ane (Cicarelli, 99.5%). BD was supplied by Sigma-Aldrich
(Cat. N°11038, purity 99%) and PD by Fluka (Cat. N°69043,
purity 99%). Reactants and solvents were distilled before the
catalytic tests. The catalysts were ground to a particle size
lower than 10 μm and the stirring rate was 1200 rpm in order
to eliminate internal and external mass transfer resistances.
The reactor was operated under the following experimental
conditions: 353 K, 20 bar hydrogen pressure, 0.188 M initial
reactant concentration (BD or PD), 61 mL reaction media
volume, 50 mg catalyst mass. The catalysts were previ-
ously reduced for 4 h at 673 K. Reactants and products were
analyzed by means of gas chromatography in a Shimadzu
GC-2010 equipped with a fame ionization detector and a
30 m long J&W InnoWax 19091 N-213 column.
The Pd, Ru and Rh activated carbon supported catalysts
were of commercial type: BASF 5Pd-C, 5Ru-C and 5Rh-C.
Before the catalytic tests, all catalysts were reduced in a
fow of hydrogen (20 mL min−1) at 673 K for 4 h.
2.2 Catalysts Characterization
The catalysts were characterized by ICP, XRD, XPS, TEM
and nitrogen physisorption.
The metal loading of catalysts was obtained by digesting
the sample and then analyzing the solution by ICP technique,
in Perkin Elmer Optima 2100DV ICP-OES equipment.
X-ray difractograms were obtained in a Shimadzu XD-1
equipment, using CuKα radiation (λ=1.5405 Å) fltered with
Ni. The 2θ range between 10 and 90° was scanned at a rate
of 1° min−1. The samples of catalysts and a typical carbon
support (RX3) were prepared by milling the material to a
fne powder and treating it in hydrogen.
The particle size of the supported metals was studied by
transmission electron microscopy (TEM). Samples of the
reduced ex situ catalyst were milled and then dispersed in
a 50% v/v ethanol:water solution for further analysis. The
measurements were performed in JEOL 100CX Transmis-
sion Electron Microscope, operated at 100 kV. For each cata-
lyst, a set of digital images were taken in order to measure
particle diameters. The Digital Micrograph software was
used. Then, size particles distributions were obtained.
XPS measurements were carried out with a Multitech
UniSpecs equipment, with a MgK (hv: 1253.6 eV) radia-
tion source and a pass energy of 50 eV. The XPS system
analysis pressure was kept at 5.10−7 Pa. Catalyst samples
were reduced ex situ for 1 h with H2 at 673 K and treated
in situ for 10 min in the instrument chamber before record-
ing the spectrum. A careful deconvolution of the spectra was
made. The areas of the peaks were estimated by calculating
the integral of each peak after subtracting a Shirley back-
ground and ftting the experimental peak to a combination
of Lorentzian/Gaussian lines of 30–70% proportions. The
reference binding energy (BE) was the C1s peak at 284.5 eV.
Brunauer–Emmett–Teller surface area (SBET) of the
prepared catalysts was determined by means of nitrogen
physisorption at 77 K following the BET model in a Quan-
tachrome Corporation NOVA-1000 equipment. Before the
measurement, the samples were degassed at 573 K in a
vacuum.
3 Results and Discussion
3.1 Characterization Results
Table 1 shows the results of metal load as determined by
ICP, the metal dispersion (D) as calculated from particle size
information obtained by TEM (dpTEM), and the XPS results.
The ICP results indicate that the metal load is in most cases
close to the theoretical value, with an error margin of 2%.
The results of TEM can be seen in Figs. 2 and 3. Figure 2
shows the micrographs of the catalyst samples in which the
diferent phases of the catalyst can be clearly distinguished.
Metals (black dots) and support (light color particles). It can
be seen in Fig. 2 that metallic particles had a spherical shape
and were dispersed throughout the support homogeneously.
Table 1 ICP, TEM and XPS results
Catalyst Metal
load
dpTEM (nm) D (%) XPS
Region BE (eV)
(wt%)
5Pt-C
5Pd-C
5Rh-C
5Ru-C
5.1
4.9
5.0
4.9
4.1
5.2
30
24
28
11
4f7/2
71.3 (56.4%)
73.2 (43.6%)
3d5/2
3d5/2
3p3/2
335.4 (54.1%)
336.7 (45.9%)
1H-NMR were obtained in a Bruker Avance II 300 MHz
spectrometer using CDCl3 as solvent. Chemical shifts are
reported in δ units and are referenced to the residual solvent
signal (7.26 ppm).
4.5
307.0 (46.4%)
308.8 (53.6%)
11.1
461.6 (44.3%)
463.1 (55.7%)
1 3

N. Carrara et al.
Fig. 2 TEM micrographs of the
catalysts. a 5Pt-C. b 5Pd-C. c
5Ru-C. d 5Rh-C
Fig. 3 Distribution of metal par-
ticle size for diferent activated
carbon supported catalysts
Figure 3 shows the histograms of metal particle size distri-
bution. The values of average metal particle size were obtained
from the TEM micrographs by using the formula for the vol-
ume/surface average:
∑
∑
ni ⋅ di3
ni ⋅ di2
dTEM
=
(1)
1 3

Selective Hydrogenation of Diketones on Supported Transition Metal Catalysts
where dpi is the particle diameter and ni is the number of
particles of diameter dpi. Table 1 shows the results of the
calculation of the average metal particle size.
catalyst has the lowest particle size (dp=4.1 nm) and hence
the highest dispersion (D = 30%). The Ru catalyst has the
biggest particle size (dp=11.1 nm) and the lowest disper-
sion (D=11%). Rh and Pd have intermediate values, with
dp=4.5 nm (D=28%) and 5.2 nm (D=24%), respectively.
The diferences in the metal particle size and metal disper-
sion could be attributed at least in part to the interaction
between the precursor and the surface groups of the carbona-
ceous support during the catalyst preparation procedure. The
metal precursors salts in solution are present as metal com-
plexes. These complexes can be tetrahedral or octahedral.
The magnitude of the molecules could afect the magnitude
of the dispersion forces between them during the impregna-
tion. If the dispersion forces are proportional to the size of
the complexes, bigger complexes would give higher disper-
sion [3]. Besides, due to electronic or geometrical efects,
diferent carboxylic groups on the carbonaceous supports
could interfere and give higher dispersion of the metallic
phase.
The metal dispersion (D) of the catalysts was calculated
from the TEM particle size values using Eq. (2).
(
[
[
]
]
[
]
)
⋅ 1010
at
m2
g
6 ⋅ ꢀ
⋅ PA
mol
D(%) =
⋅ 100%
(2)
[ ]
⋅ dp Å ⋅ NA
g
m3
ꢁ
where σ: atomic surface density [at m−2]; PA: atomic weight
of the metal [g mol−1]; ρ: density of the metal [g m−3]; dp:
particle diameter (average) [Å]; NA: Avogadro´s Number
[17]. The model particle of Sundquist [18] was used, with
a mixture of 70% < 111 > planes, 25% < 100 > planes and
5% < 110 > planes. This model gives surface densities of
14.2, 14.5 and 15.5 atoms nm−2 for Pt, Pd and Rh, consider-
ing fully centered cubic structures (fcc). These values were
mainly taken from the review of Scholten et al. [17]. In the
case of Ru the most common crystal habitat is hexagonal
close packed (hcp) though fully centered cubic crystals are
also found in supported systems [19]. Calculating for fcc Ru
(cell size a=3.83 Å) with Sundquist’s model gives an atomic
surface density of 14.91 atoms nm−2.
Figure 4 shows the XPS spectra of the Pd and Rh 3d
regions of the 5Pd-C and 5Rh-C catalysts, the Ru 3p region
of the 5Ru-C catalyst and the Pt 4f region of the 5Pt-C cata-
lyst pretreated in hydrogen at 673 K. Figure 4 shows the Pd
and Rh 3d5/2 and 3d3/2 signals separated by approximately
5.25 and 4.75 eV, and the 4f5/2 and 4f7/2 signals of Pt sepa-
rated by 3.35 eV, in total accordance with literature val-
ues [20, 21]. The points are the experimental data and the
curves beneath are the corresponding deconvoluted peaks.
The XPS results of the catalysts can be found in Table 1.
Table 1 shows the results of the calculation of the aver-
age particle diameter (dpTEM) and the metal dispersion (D),
of the diferent catalyst samples. The order found for the
particle sizes was: 5Pt-C<5Rh-C<5Pd-C<5Ru-C. The Pt
Fig. 4 XPS spectra of the Pt 4f,
Pd and Rh 3d, and Ru 3p region
of all catalysts
1 3

N. Carrara et al.
The textural properties of the support and the catalysts
were also assessed by means of nitrogen physisorption. Type
IV isotherms were got indicating the presence of hysteresis
phenomena, typical of microporous solids. The pore distri-
bution was obtained by means of the method Barrett-Joyner-
Halenda method. The specifc surface area, the pore size and
the pore volume are indicated in Table 2. All metal catalysts
had similar textural properties and only the Rh catalyst had
a lower surface area and bigger average pore diameter than
Fig. 5 X-ray difractograms of the RX3 support and the metal cata-
lysts
The deconvolution of the metal (Me) 3d5/2, 3p3/2 and 4f7/2
spectra resulted in two peaks for all the samples, indicating
the presence of two diferent Me species on the catalysts:
a metal reduced Me° specie and other electrodefcient one
Meδ+, with δ=2 for Pd or Pt, while δ=3 for Rh and Ru [3,
21]. For the cases of Pt and Pd, the major species was the
reduced one, with values of 56.4% and 54.1%, respectively.
In the case of Rh and Ru, the concentrations of reduced spe-
cies (46.4 and 44.3%) were the lowest.
The X-ray difractograms of the support and catalysts are
shown in Fig. 5. In all the difractograms a large peak at
26.4º is seen, that corresponds to C<002>phase of graph-
ite [16]. Then an intense peak at 2θ = 39.9º of the 5Pd-C
catalyst was attributed to the<111>plane of metallic Pdº
[22]. In the 5Pt-C catalyst a small peak appears at 2θ=39.7º,
which would correspond to the<111>plane of Pt [23–25].
For 5Rh-C catalyst, a peak of the<111>plane of Rh was
observed at 2θ=41.2º. In the case of the 5Ru-C catalyst, an
intense peak of the<101>main plane of Ru was observed
at 2θ = 43.3º, as reported by other authors [26]. The low
intensity in 5Pt-C and 5Rh-C catalyst difractograms could be
attributed to the small metal particle size observed by TEM.
Table 2 N2 physisorption results
Catalyst
BET surface
dpore (nm)
Vpore (cm3 g−1
)
(m2 g−1
)
5Pt-C
5Pd-C
5Rh-C
5Ru-C
896
809
360
797
2.4
2.6
7.3
2.8
0.59
0.56
0.54
0.57
Fig. 6 Results for BD hydrogenation. a Total conversion, b Selectiv-
ity to AC and c Selectivity to BDO
1 3

Selective Hydrogenation of Diketones on Supported Transition Metal Catalysts
Table 3 Turnover frequencies (TOF) in the hydrogenation of BD and
Table 4 PD hydrogenation results. Selectivity values at an isoconver-
PD
sion condition of 50%
BD
PD
TOF (s−1
Catalyst Solvent
Time (min)
S(2H3P + 3H2P)
(%)
Sothers (%)
Catalyst TOF (s−1
)
rº (mol.
min−1
)
rº
−1
cat
−1
cat
g
)
(mol min−1
g
)
5Pt-C
5Rh-C
5Ru-C
5Pd-C
5Pd-C
Cyclohexane
38
43
145
50
81
75
66
90
85
19
25
34
10
15
Cyclohexane
Cyclohexane
Cyclohexane
Isopropanol
5Pt-C
5Rh-C
5Ru-C
5Pd-C
0.67
0.38
0.66
0.37
0.0031
0.0031
0.0021
0.0026
0.57
0.34
0.39
0,31
0.0027
0.0028
0.0013
0.0021
80
Others: PDO, esters, heavy products
Reaction conditions: Cºdiketone =0.188 M, Wcat=50 mg, V=61 mL,
T=353 K, PH2 =20 bar, Solvent: Isopropanol
Figure 6c shows values of selectivity to the vic-diol BDO
during the hydrogenation of BD. The most selective catalyst
was Ru, with ca. 80% at 8 h reaction time. This high selectiv-
ity was attributed in part to the high amount of surface Ru3+
species, that favors the adsorption of AC and to the capacity
or Ru for hydrogenating C = O bonds [3]. Ru3+ is a hard
Lewis acidic site and due to its high density charge would
favour the adsorption of a carboxylic group of the α-diketone
(Lewis basic site) and would favors its total hydrogenation to
the butanediol. In the case of Pt and Rh, a growing selec-
tivity can be seen. Pd was the least selective to the over
hydrogenated product.
the others, this could be attributed to diferent carbonaceous
supports of the commercial catalysts tested.
3.2 Catalytic Activity Results
Some blank tests of BD and PD hydrogenation without cata-
lyst were made in order to check any interference from the
reaction vessel and pipelines. Conversion was lower than 1%
after 8 h reaction time.
Results of BD catalytic hydrogenation can be seen in
Fig. 6 and Table 3. As seen in Figure 6a, all catalysts were
active in BD hydrogenation. The total conversion followed
the order: Pt > Rh > Pd > Ru. The activity pattern matches
the results of obtained metal dispersion, or metal particle
size. With high dispersion and the small diameter parti-
cle, the total conversion of BD was increased, eventhough
electronical or geometrical efects of the carbonaceous
support could not be discarded as the support specifc
properties, such as the superfcial groups or texture and
porous structure could also modify the adsorption–des-
orption equilibrium of the reactants and products, and so
forth.
These results for BD hydrogenation are similar to those
obtained previously with metal catalysts based on composite
polymer-alumina supports [3, 5, 6]. They also correlate with
other reports in the literature [9]. The Pd seems to be the
most appropriated phase for obtaining self-hydrogenation
product AC, while the other favors the over hydrogenation,
especially Ru.
Tables 3 and 4 show the results of PD hydrogenation.
In Table 3 TOF values for PD hydrogenation can be seen.
Table 4 has selectivity values corresponding to points with
50% conversion. Since PD is an asymmetric diketone, the
product of partial hydrogenation can be 3-hydroxy-2-pen-
tanone (3H2P) or 2-hydroxy-3-pentanone (2H3P). In our
analysis we have considered the selectivity to the interme-
diate products as the sum of the selectivity to these two iso-
mers (S(2H3P+3H2P)). The selectivity to other reaction prod-
ucts was indicated as Sothers. The results indicate that the
selectivity to the intermediate products followed the order
Pd>Pt>Rh>Ru. These results show that the most promis-
ing active phase for producing the intermediate products is
Pd. At 50% of PD total conversion, S(2H3P+3H2P) for Pd was
90% when cyclohexane was the solvent and 85% when iso-
propanol was used, indicating a slight negative infuence of
the polarity of the solvent over the selectivity, but a notable
negative infuence on the conversion.
Values of turn over frequencies (TOF) are presented in
Table 3. They were calculated using values of metal disper-
sion and the initial reaction rates. The specifc activity of a
catalytic center per unit time for BD hydrogenation indicates
that the order was Pt≫ Ru > Rh ≈ Pd. Despite Ru catalyst
has the poor dispersion, the good performance per site was
attributed to the high interaction with C=O bond, in coinci-
dence with Pt [27]. Pd and Rh catalysts showed lower levels
of activity per site for BD hydrogenation.
In Fig. 6b the selectivity to AC for the diferent catalysts
was observed. The most selective catalyst was Pd. It can
be seen that for this catalyst the selectivity is high in the
frst 120 min of reaction (SAC = 98%) and then it slightly
decreases to 95% up to 8 h. Pt and Rh catalysts were less
selective to AC, especially after 8 h reaction time, they had
selectivity values of 59% and 45%, respectively. The Ru
catalyst was the least selective to AC.
The conversion order was identical during the hydrogena-
tion of BD or PD, irrelevant of the simetry of the substrate,
suggesting that in part small metal particle sizes favor the
1 3

N. Carrara et al.
hydrogenation rate. In terms of activity, Pt and Rh were the
most active, followed by Pd, and fnally, Ru. When the efect
of the solvent is analyzed it can be seen that the reaction
proceeds with a higher rate when cyclohexane is used as
solvent. The efect of solvent in reaction rate could be attrib-
uted to solvent polarity. The polar solvents could be absorb
over metal active sites, and compete with substrate and H2
adsorption, reducing the reaction rate [3].
the hydroxyketone product, being the addition of an H atom
the rate limiting step [5].
Pd is a metal frequently recommended for performing the
hydrogenation to intermediate products during the reaction
of diketones [31, 32]. The loss of selectivity of Pt, Rh and
Ru catalyst could be associated to over hydrogenation capac-
ity. The diferences found in selectivity during the hydro-
genation of BD and PD can be attributed to the diferent
capacities of adsorption of the C= O double bond. Pt, Ru
and Rh easily chemisorb C=O through their π electrons. Pd
has however a weaker interaction with this type of bonds.
The alpha diketones coexist in two isomeric forms: the keto
form (with two C=O bonds in the same molecule) and the
enol form (with one C=O and one C=C bond) as shown
in Fig. 7 [33–35].
Table 3 shows TOF values of PD hydrogena-
tion for the different catalysts. The order obtained was
Pt≫Rh>Ru≈Pd. Just like BD hydrogenation, Pt shows the
high activity per site. Rh, Pd and Ru present lower values.
Gertosio et al. [28] found similar results when hydrogenat-
ing a reactant with alpha-diketonic functional groups in a
C11–C12 position. They used novel metal supported on SiO2
and carbon, and found that the order of activity when com-
paring the initial rate was: Pt>Rh>Ru>Pd. In that work
the Pd catalyst had an induction time higher than other met-
als; after activating, the metal conversion was complete. In
our work, similar results were obtained, but no induction
time was observed for any catalyst. This could be due to
many reasons, like solvent efect, catalyst properties and/or
diferences in the thermal reduction treatment [29]. In our
case, using a carbonaceous support favors the reduction of
metallic active sites.
In Fig. 8, 1H-NMR spectroscopy results of both dike-
tones BD (Fig. 8a) and PD (Fig. 8b) are presented. The PD
spectrum shows a signal between 5 and 5.25 ppm, which
corresponds to a quartet; according to the multiplicity and
chemical shift, this signal could be assigned to the enol form
of PD. The absence of this signal for BD is due to enol form
is below detection range of the technique. In other words, the
keto-enol equilibrium of BD is much more displaced to the
keto-form, and therefore the concentration of enol tautomer
is below the detection limit of the experiment [36, 37].
The enol form has a C = C double bond that can be
easily chemisorbed by any of the employed Mδ+ active
phases, especially Pd, beside sits weaker interaction with
C = O bonds, it could explain the high selectivities to
α-hydroxyketones [38, 39].
The dissociative chemisorption H2 is done on reduced
metallic sites Me°. The amount of available d electrons in
the catalyst set was: Pd (d10) > Pt (d9) > Rh (d8) > Ru (d7).
The rupture of H–H bond is favorable on sites with more
d available electrons [30]. The selectivity pattern achieved
in the hydrogenation of the diketones BD and PD could be
associated to the electronic density in the external d orbital.
It has been reported that the hydrogenation of BD on Pd sup-
ported over composite supports, gives high selectivities to
The hydrogenation of alpha- and beta-unsaturated alde-
hydes and ketones to saturated aldehyde or ketone, using
noble metal supported catalysts were reported by Ide et al.
[40]. These authors found that in terms of activity Pd was
the phase with the best turnover frequencies. The Pd phase
Fig. 7 Keto-enol equilibrium of
a generic diketone and proposed
mechanism of consecutive
reactions
1 3

Selective Hydrogenation of Diketones on Supported Transition Metal Catalysts
Fig. 8 1H-NMR Spectra of a BD and b PD
Fig. 9 Scheme of the Horiuti–Polanyi reaction mechanism as applied to the hydrogenation of ketones over supported Pd catalysts. Hydrogena-
tion of BD in the enol isomeric forms
was the most active phase for C = C hydrogenation, but
failed in the hydrogenation of C = O for producing unsa-
tured alcohols.
the last step the addition of the H atom to the C1 atom occurs
with the subsequent desorption of the molecule (Fig. 9).
The high selectivity of Pd toward the hydroxyketone
product can be explained by the occurrence of the Horiuti-
Polanyi mechanism [41]. Over the Pd0 species dissociative
chemisorption of hydrogen frst occurs. On nearby Pd elec-
trodefcient atoms (Pdδ+) the C=C moiety in its enol iso-
meric form gets adsorbed via a π bond. Then one H atom
forms a σ bond with one of the carbons (C2) while the other
one (C1) remains adsorbed forming a σ bond with Pdδ+. In
4 Conclusions
The amount of PD in the enol form, observed by 1H-NMR
spectroscopy, was ca. 3 wt%; while the keto-enol equilib-
rium of BD could not be detected because the concentra-
tion of the enol form was below the measurement detection
limits. The Pd phase has better interaction with C =C than
1 3

N. Carrara et al.
12. Campisciano V, Gruttadauria M, Giacalone F (2019) Chem-
CatChem 11:90–133
C = O bonds, besides the synergy of enol presence, good
C = C adsorption and H2 chemisorption leads to a high
α-hydroxyketone selectivity for both reactions. On the
other hand, the other metal active phases (Pt, Rh and Ru)
showed lower selectivity to α-hydroxyketones especially
at higher values of reaction time, when the formation of
vic-diols were favored. This was especially true for Ru
which was the most selective metal catalyst for getting the
double hydrogenation product. As similar reaction mecha-
nism occured for BD and PD, the asymmetry structure
did not afect the pattern. The kind of solvent seemed to
be slightly relevant for the selective hydrogenation of PD
with Pd catalysts as the selectivity to the sum of hydrox-
yketone products in diferent solvents varied only ca. 5%.
In terms of activity, the reaction was faster in cyclohexane
phase than in isopropanol, indicating less interaction of the
Pd catalyst with apolar solvents.
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Acknowledgements We thank Consejo Nacional de Investigaciones
Científcas y Técnicas (CONICET), Agencia Nacional de Promoción
Científca y Tecnológica (ANPCyT) and Universidad Nacional del
Litoral (UNL) for the financial support of this work (Grants PIP
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Compliance with Ethical Standards
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N (2003) Appl Surf Sci 210:222–230
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