Chiral Pt/ZrO2 catalysts. enantioselective hydrogenation of 1-phenyl-1,2-propanedione

Article abstract of DOI:10.3390/molecules15053428

The enantioselective hydrogenation of 1-phenyl-1,2-propanedione over Pt colloids stabilized with (R,S)-4,5-dihydro-4,5-diphenyl-2-(6-cyanopyridinyl) imidazoline (CI) supported on a meso-structured ZrO₂ under a pressure of 40 bar of H₂ at 298 K has been investigated. The metal loading in all catalysts was 1 wt%. The effect of the amount of chiral modifier on the metal particle size and on the catalytic behavior was analyzed. It was found that as the CI/Pt molar ratio increases from 2.5 to 3.5 the Pt crystal size decreases from 3.0 to 1.8 nm. All catalysts were very active in the studied reaction, with the most active one being the catalyst with smaller Pt particles, whereas the selectivity is higher in those catalysts with larger chiral modified Pt metal particles.

Full text of DOI:10.3390/molecules15053428

Molecules 2010, 15, 3428-3440; doi:10.3390/molecules15053428
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molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Chiral Pt/ZrO2 Catalysts. Enantioselective Hydrogenation of
1-phenyl-1,2-propanedione
Claudia Urbina 1, Cristian Campos 1, Gina Pecchi 1, Carmen Claver 2 and Patricio Reyes 1,*
1
Facultad de Ciencias Químicas, Universidad de Concepción, Casilla 160-C, Concepción, Chile;
E-Mails: curbina@udec.cl (C.U.); gpecchi@udec.cl (G.P.); ccampos@udec.cl (C.C.)
2
Departamento Química Física i Inorgànica, Universitat Virgili i Rovira, Marcel.lí Domingo, s/n.
43007 Tarragona, Spain; E-Mail: carmen.claver@urv.cat (C.C.)
 Author to whom correspondence should be addressed; E-Mail: preyes@udec.cl.
Received: 13 February 2010; in revised form: 15 April 2010 / Accepted: 10 May 2010 /
Published: 12 May 2010
Abstract: The enantioselective hydrogenation of 1-phenyl-1,2-propanedione over Pt
colloids stabilized with (R,S)-4,5-dihydro-4,5-diphenyl-2-(6-cyanopyridinyl)imidazoline
(CI) supported on a meso-structured ZrO2 under a pressure of 40 bar of H2 at 298 K has
been investigated. The metal loading in all catalysts was 1 wt%. The effect of the amount
of chiral modifier on the metal particle size and on the catalytic behavior was analyzed. It
was found that as the CI/Pt molar ratio increases from 2.5 to 3.5 the Pt crystal size
decreases from 3.0 to 1.8 nm. All catalysts were very active in the studied reaction, with
the most active one being the catalyst with smaller Pt particles, whereas the selectivity is
higher in those catalysts with larger chiral modified Pt metal particles.
Keywords: colloidal platinum; mesostructured ZrO2; enantioselective hydrogenation;
chiral modifier
1. Introduction
In the last few years, the applications of single enantiomers of chiral compounds have increased
significantly, especially for their uses in the field of pharmaceuticals, agrochemicals, and flavors and
fragrances [1]. Even though homogeneous metal complexes with chiral ligands are the most widely
used catalysts, heterogeneous catalysis has emerged as an important alternative since Orito et al [2]

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reported the enantioselective hydrogenation with ee’s up to 95% of ethyl pyruvate to (R)-ethyl lactate
over cinchonidine-modified Pt catalysts, Other research teams have evaluated these catalytic systems
in the mentioned reaction [3–4] using Pt, Pd and Ir-supported on SiO2, Al2O3 and others [5–9]. Indeed,
this reaction is presently a sort of test reaction in the field of asymmetric catalysis.
However, the reaction becomes more difficult, both in terms of enantio- and regioselectivity (rs),
when the substrate has more than one center to be hydrogenated, such as during dione hydrogenation
in 1-phenyl-1,2-propanedione [10–12]. As far as regioselectivity is concerned, the product of interest
is (R)-1-phenyl-1-hydroxy-2-propanone, which has interesting applications in the synthesis of
compounds used as precursors for hypertension and asthma treatments, among others [13,14].
Supported platinum catalysts have been widely studied for this type of applications [15], and it has
been found that metal particle size plays an important role in asymmetric synthesis when performed in
the presence of cinchonidine as a chiral modifier (CM). The most promising catalytic behavior has
been obtained for catalysts with a metal particle size around 3.0 nm and metal loading close to 5 wt % .
The catalysts containing lower metal loadings usually display smaller particle sizes, and therefore are
inappropriate for this purpose. However, using colloidal preparation procedures, metallic crystals with
different average particle sizes can be prepared. Thus, even with low metal loading, catalysts with
higher metal metal particle size can be obtained. In fact, it is well known that metal colloids with
different particle sizes can be prepared by this procedure in the presence of a chiral modifier, which
may lead to a preferential hydrogenation to (R)-ethyl lactate during the hydrogenation of ethyl
pyruvate [16,17]. Metal crystals stabilized with chiral complexes used as catalysts in different
reactions have been widely reported [18–23].
Support selection is relevant in the preparation of heterogeneous catalysts for the enantioselective
hydrogenation of -ketoesters, -functionalized ketones and ,-diketones [24]. Several groups have
tried to improve the hydrogenation of ethyl pyruvate by preparing cinchonidine-modified Pt catalysts
on various supports. Hall et al. [25] described the use of a Pt/MCM-41 catalyst that exhibits low
reaction rates due to mass transfer limitations and enantiomeric excess up to 64%. Zeolites of different
structures and acidities can also display highly enantioselective hydrogenation supported on Pt
catalysts. In fact, Böhmer et al. [26] have reported the re-use of Pt/HNaY zeolite in the hydrogenation
of ethyl pyruvate in acetic acid and cyclohexane reaching a high e.e. (88%). Additionally, e.e. up to
75% was reported for Pt/clay catalysts (28% e.e. for Rh/K-10)[27], while e.e.s below 35% were
described for Pt/C catalysts, despite improvement by oxidative treatment [28]. In the last few years,
research has focused on improving the support’s structural and morphological properties to enhance
the catalytic behavior [29–31]. One of the main advantages of these structured solids compared to the
traditional supports is that they can produce a confinement effect that can favorably affect both activity
and the selectivity in different reactions.
Thus, the use of Pt on mesoporous MCM-41 in the enantioselective hydrogenation of 1-phenyl-1,2-
propanedione was successfully demonstrated. Toukoniitty et al. [29] have studied three different Pt
metal contents (5, 10 and 15 wt % ) on MCM-41 modified with (-)-cinchonidine (CD), obtaining a
maximum enantiomeric excess of (R)-1-phenyl-1-hydroxy-2-propanone close to 44% using the 15 wt %
Pt/MCM-41 catalyst. This behaviour was attributed to the larger metal particle size displayed by this
catalyst. Reyes et al. [30] reported similar results in terms of activity and enantioselectivity using a
1 wt % Pt/MCM-41, which is more active than a 1 wt%Pt/SiO2 catalyst with comparable metal particle

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size. The observed behavior was explained in terms of the confinement effects produced in the MCM-
41 hexagonal channels. Thus, ZrO2 has shown important properties from the catalytic point of view
and ZrO2 films on a metal surface have also been extensively used as a protection agent against
corrosion [31]. Oxide gels in the presence of a surfactant or appropriate templates produce nanotubes,
and thus ZrO2 is mesoestructured when prepared from a surfactant, such as cetyltrimethylammonium
bromide (CTMABr), which acts as template for nanotubes with a given channel diameter [32]. ZrO2
nanotubes may also be obtained by covering carbon nanotubes with oxide gels followed by calcination
[33]. These solids display high surface areas and are materials with potential catalytic applications
[34]. Two types of chiral modifiers have been widely studied: tartaric acid used as modifier of Ni
catalysts [35] and cinchona alkaloid and analogs, where cinchonidine (CD) [36,37] is the most
representative example. However, other chiral ligands may also display interesting behavior.
In
the
present
work,
we
have
prepared
(R,S)-4,5-dihydro-4,5-diphenyl-2-(6-
cyanopyridyl)imidazole, which is an imidazoline derivative used as a chiral modifier (see Figure 1).
This ligand has been used by Claver et al. [38,39] to prepare a Pd complex with applications in
fine chemistry.
Figure 1. Structure of (R,S)-4,5-dihydro-4,5-diphenyl-2-(6-cyanopyridyl)imidazole.
In this work, Pt-supported catalysts in which the active phase was obtained from a colloidal
dispersion, followed by impregnation on nanostructured ZrO2 support were synthesized, [33,34]. The
colloid dispersion was prepared by reduction of a Pt precursor in the presence of the previously
mentioned ligand used as chiral modifier and also as stabilizer of the colloid, because it limits the
excessive growing of the metal nanoparticles. The prepared catalysts were characterized and evaluated
in the batch hydrogenation of 1-phenyl-1,2-propanedione (Figure 2) at 298 K and 40 bar of H2 using
cyclohexane as solvent.
2. Results and Discussion
2.1. Characterization
X-ray diffraction patterns of the prepared catalysts do not show any diffraction line characteristic of
monoclinic and tetragonal phases and no low angle XRD reflections. Similar results have been found
by Larsen et al. [39] and Blin et al. [40], who described the synthesis of sulfated mesoporous zirconia

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mediated by lauryl sulfate in alkaline solution and CTMABr, respectively. In contrast to the synthesis
of silica mesoporous structured materials, these authors suggest that the pore formation mechanism
does not imply detectable bi- or three-dimensional X-ray ordering. Indeed, even if the low angle signal
is not observed in the XRD-pattern, samples exhibit a uniform pore size distribution in the mesoporous
range. In the present study, in agreement with Blin [40], the absence of a small angle reflection line
can be attributed to a relatively large pore size distribution. Additionally, the absence of a diffraction
line due to Pt may be explained by the small size of some metal nanoparticles (shown by
TEM analysis)
The nitrogen adsorption-desorption isotherms correspond to type IV in the BDDT classification
[41]. The isotherm’s adsorption branch can be decomposed in three parts: the monolayer-multiple
adsorption of nitrogen, the capillary condensation of nitrogen within the mesopores, and then
saturation. The hysteresis loops reveal the presence of open capillaries characteristic of MCM-41 type
of materials. However, in the CNTsZrO2, the pore size distribution is centered in the range 2.5 to 4 nm
and display a BET surface area of 87 m2 g-1 and a pore volume of 0.18 cc g-1.
TEM micrographs of the calcined ZrO2 materials show, for the Pt/CNTsZrO2 samples, typical
shapes of nanostructured solids with a regular hexagonal array of mesoporous channels, results that are
in good agreement with those previously reported by Beck et al. [42] as can be seen in Figure 2.
Figure 2.Transmission electron micrographs of Pt(2.5)/CNTsZrO2 catalysts.
50 nm
However, the study of different areas of the samples with TEM resulted in the detection of
amorphous areas in the solid. The presence of Pt crystals located inside the channel of structured
supports can be clearly observed, with a very narrow metal crystal distribution, in the range 1 to 4 nm.
As the ligand/Pt molar ratio increases, the average decreases from 3.0 to 1.8 nm. Using a same
preparation procedure, if no ligand is added during the reduction of platinum precursor, the colloid is
not stable; therefore as comparison, in absence of modifier, the catalyst was prepared by impregnation
of chloroplatinic acid followed by calcinations at 300 ºC and reduction in hydrogen at 500 ºC. The
particle size was 3.8 nm. A summary of the characteristic of the studied catalysts is given in Table 1.

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Table 1. Platinum particle size(d) obtained by TEM and metal dispersion (D) evaluated
from TEM results of Pt(x)/CNTsZrO2 catalysts.
Catalysts
dTEM , nm
DTEM
0.24
0.51
0.35
0.31
Pt(0.0)/CNTsZrO2
Pt(2.5)/CNTsZrO2
Pt(3.0)/CNTsZrO2
Pt(3.5)/CNTsZrO2
3.8
1.8
2.6
3.0
2.2. Catalytic activity in 1-phenyl-1,2-propanedione hydrogenation
Figure 3 shows a scheme of the enantioselective hydrogenation of 1-phenyl-1,2-propanedione. The
reaction was studied at 25 ºC and 40 bar of H2 on zirconia-supported modified-Pt catalysts.
Figure 3. Reaction scheme of the hydrogenation of (A) 1-phenyl-1,2-propanedione. (B)
(R)-1-hydroxy-1-phenyl-2-propanone; (C) (S)-1-hydroxy-1.phenyl-2-propanone; (D) (S)-2-
hydroxy-1-phenyl-1-propanone; (E) (R)-2-hydroxy-1-phenyl-1-propanone; (F) (1R,2S)-1-
phenyl-1,2-propanediol; (G) (1S,2S)-1-phenyl-1,2-propanediol; (H) (1S,2R)-1-phenyl-1,2-
propanediol; (I) (1R,2R)-1-phenyl-1,2-propanediol.
Using these catalysts, it was found that a preferential formation of (R)-1-phenyl-1-hydroxy-2-
propanone can be induced. The evolution of the conversion level over time for the studied catalysts
present similar trends for all catalysts and are compatible with a pseudo first-order reaction, showing
differences in activity depending to metal loading, where the regioisomers (R) and (S)-1-hydroxy-2-
phenylpropanone are the main products.

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Figure 4. Evolution of the conversion level with time during 1-phenyl-1,2-propanodione
hydrogenation on Pt/CNTsZrO2 catalysts.
This behavior may be explained mainly in terms of electronic factors. The aromatic ring contributes
to polarization to a greater extent the C=O bond close to the ring, and therefore the reactions lead to
(B) (R)-1-hydroxy-1-phenyl-2-propanone and (C) (S)-1-hydroxy-1.phenyl-2-propanone, whereas the
regioisomers (D) (S)-2-hydroxy-1-phenyl-1-propanone and (E) (R)-2-hydroxy-1-phenyl-1-propanone
are not favored.
Figure 5 shows the kinetics of products formation in a batch reactor in the 1-phenyl-1,2-
propanedione hydrogenation on one representative catalyst, Pt (3.0) /CNTsZrO2 catalyst. The results
indicate that under the experimental conditions, the catalyst displays a high activity, with a conversion
level close to 90% at 1,800 min, moderate ee (30%) and complete regioselectivity. The evolution of
the dione concentration and the other reaction products follow the expected trends: the dione
concentration decreases through a first-order law, leading to hydroxyketones, (R)-1-hydroxy-1-
phenylpropanone and (S)-1-hydroxy-(R)-1-phenylpropanone as intermediate compounds (B and C in
Figure 3). A maximum reaction time of close to 125 min is obtained for the most active catalyst
(Pt/CNTsZrO2). A longer time to reach a maximum hydroxyketone concentration was observed for the
other catalysts: 105 and 150 min for Pt/MSZrO2 and Pt/ZrO2 respectively (not shown). All studied
catalysts were completely regioselective; no products generated by the hydrogenation of the C=O bond
located far from the aromatic ring were obtained: (R)-2-hydroxy-1-phenylpropanone and (S)-2-
hydroxy-1-phenylpropanone (E and D, respectively, in Figure 3). In all cases, the presence of over-
hydrogenated diol products was observed, and the evolution of the diol concentrations follow the
expected sigmoid curve, characteristic of final products of consecutive reactions. Thus, the reaction
pathway may be represented by irreversible, parallel and consecutive reactions.

Molecules 2010, 15
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The first-order reaction rate constants, denoted as kg for dione consumption, can be easily obtained
from log CAo/CA vs. t plots. The reaction constants k1R and k1S, corresponding to the formation of (R)-
1-hydroxy-1-phenylpropanone and (S)-1-hydroxy-(R)-1-phenylpropanone, may be obtained from the
global first order constant and the ratio of the concentration of these compounds (B and C in the
scheme), that is CB/CS. Additionally, the reaction constants k2R and k2S, associated to the consecutive
reactions, can be evaluated from the well-known equations for intermediates of consecutive reactions,
in terms of Cmax and tmax, and are the maximum concentration and the time required to get that level of
the corresponding intermediates, respectively. The obtained results are summarized in Table 2.
Figure 5. Evolution of the concentration of the substrate and different reaction products
during 1-phenyl-1, 2-propanodione hydrogenation. T = 25 ºC, P H2= 40 bar, catalyst
Pt(3.0)/CNTsZrO2 weight 100 mg and CD concentration = 0.25 mM.
The rate constants for the first and second hydrogenation steps (k1 and k2) increase as the molar
ratio ligand/Pt increases. This can be explained by the fact that the catalyst prepared in the presence of
higher ligand/Pt molar ratio possesses higher metal dispersion and consequently higher hydrogenation
rates. In all cases, the confinement effect produced by the support provides an enhancement in the
reaction rate compared to a non-structured support [15]. The presence of the chiral ligand on the
catalyst surface favored the pathway to produce the R-intermediate (k1R > k1S); in the second step, the
hydrogenation of the S-intermediate also improved (k2S > k2R).
The reaction performed in absence of ligand (on the catalyst) does not show enantiomeric
enhancement, confirming that a chiral molecule is necessary for the generation of modified catalytic
sites responsible of the production of a given enantiomer. The enantiomeric excess displayed by the
catalysts does not significantly change with the conversion level, although it is affected by the
ligand/Pt molar ratio used in the preparation of the catalysts. As this ratio increases from 2.5 to 3.5, the

Molecules 2010, 15
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Pt particle size decreases. Consequently, the ligand remaining linked to the metal surface also
increases but not in the right position. Therefore, the platinum surface should lie parallel to the
aromatic ring, leading to a decrease in the ee (see Figure 6).
Table 2. First order reaction rate constants, in the hydrogenation of 1-phenyl-1, 2-
propanodione at 298 K on Pt(x)/CNTsZrO2 catalysts.
k1g /
102± 0.2 102
min-1
4.4
k1R
/
k1S
/
k2R
/
k2S /
102± 0.2 102
min-1
Catalyst
102± 0.2 102
102± 0.2 102
102± 0.2 102
min-1
0.98
min-1
2.2
2.8
3.3
4.0
min-1
2.2
1.9
1.9
2.0
Pt(0.0)/CNTsZrO2
Pt(2.5)/CNTsZrO2
Pt(3.0/CNTsZrO2
Pt(3.5)/CNTsZrO2
0.98
1.02
1.13
1.10
4.7
5.2
6.0
0.44
0.59
0.60
k2R = k2R  k2R ; k2S = k 2S  k2S
Figure 6. Interaction diagram Pt with (R,S)-4,5-dihydro-4,5-diphenyl-2-(6-cyano-
pyridyl)imidazole in the stabilization of metal nanoparticles.
Even though the obtained ee values are moderate, which confirms the difficulty of this reaction to
induce chirality, they are not too far from the best reported values for enantioselectivity in the same
reaction [7–12]. The fact that the three modified catalysts display almost no change in the ee values for
the hydrogenation of 1-phenyl-1,2-propanodione, close to 30%, may be understood, considering that,
apart from an optimum modifier concentration, the catalysts require a given metal particle size.
The increases in the rate constant for the first and second hydrogenation steps (k1 and k2) with the
ligand /Pt molar ratio may be attributed to two different effects: i) the catalyst prepared in the presence
of higher ligand/Pt molar ratio possesses higher metal dispersion and consequently higher
hydrogenation rates and ii) the presence of a higher proportion of ligand anchored to metallic sites in
the catalysts occurred to a greater extent in the modified sites, which are even more active than the
metallic sites. In all cases, the confinement effect produced by the support enhances the reaction rate in
comparison with a non-structured support [26]. The presence of the chiral ligand on the catalyst

Molecules 2010, 15
3436
surface favored the pathway that produces the R-intermediate (k1R > k1S); in the second step, the
hydrogenation of the S-intermediate is also improved (k2S > k2R).
When the reaction is performed in absence of a chiral modifier on the catalyst surface, the reaction
takes place without enantiomeric enhancement, and therefore CM contributes to the generation of
modified catalytic sites that are enantioselective and more active than the metallic sites.
3. Experimental
3.1. Support preparation
Carbon nanotubes were prepared following a procedure already reported [43]. Carbon nanotubes
were mixed with an aqueous solution of ZrO(NO)3. This suspension was kept under ultrasonic
vibrations for 10 min and then under boiling conditions for 3 h to evaporate the solvent. The final solid
was calcined in Ar at 600 ºC for 2 h. A grey powder was obtained and was then calcined at 600 ºC in
air for 2 h to remove the CNTs, resulting in a white powder. This support was labeled as CNTsZrO2.
3.2. Synthesis of the chiral ligand
The chiral pyridinylimidazoline ligand, (R,S)-4,5-dihydro-4,5-diphenyl-2-(6-cyanopyridyl)-
imidazoline was prepared from 2,6-dicyanopyridine (0.77 mol), which was treated with meso-1,2-
diphenylendiamine (0.77 mol) in chlorobenzene (5 mL) in the presence of ytterbium(III)
trifluoromethanesulfonate [Yb(OTf)3, 0.14 mole]. The mixture was stirred for 24 h under reflux. The
1
product was characterized by H- and 13C-NMR in a multinuclear Bruker AC instrument at
200/50 MHz using CDCl3 as solvent. The same signals and chemical shifts reported by Claver et al.
were found [38].
3.3. Catalysts preparation
Colloidal Pt was prepared as follows. An aqueous solution containing 0.60 mmole of H2PtCl2
dissolved in 160 mL of water was heated to 100 ºC, then a solution of the 0.1 mol L-1 ligand in
HCOOH was added, and reflux maintained for 60 min The amount of this later solution was varied
according the desired ligand/Pt molar ratio. The solution shows initially a yellow color; due to the
reduction of Pt (IV) to Pt(0), the solution became dark, becoming black after 10 min. Once the
colloidal dispersion was obtained, it was slowly cooled down to room temperature, washed with a
saturated solution of NaHCO3 (100 g L-1) and filtered. The amount of ligand added during the
preparation of colloidal platinum to stabilize the Pt-colloid ligand with ligand/Pt molar ratios of 0, 2.0,
3.0 and 3.5 respectively. Subsequently, a colloid dispersion containing the amount of Pt required to get
a Pt constant of 1 wt % was contacted with the support in a rotary evaporator at 308 K under stirring
for 1 h and then the solvent was removed under vacuum at the same temperature. The samples were
dried at 120 ºC for 2 h. Catalysts were labeled as Pt(x)/CNTsZrO2 (x is the ligand/Pt molar ratio). The
catalyst Pt(0)/CNTsZrO2 was prepared by impregnation of chloroplatinic acid in CNTsZrO2 followed
by calcination at 300 ºC and reduction in hydrogen at 500 ºC.

Molecules 2010, 15
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3.4. Characterization
Nitrogen adsorption at 77 K was measured in a Micromeritics ASAP 2010 apparatus. TEM
micrographs were obtained in a Jeol Model JEM-1200 EXII System, X-ray diffraction studies were
performed in a Rigaku diffractometer.
3.5. Hydrogenation reaction
The hydrogenation of 1-phenyl-1,2-propanedione was studied at 298 K and 40 bar of H2 with
0.01 mol L-1 of substrate, 100 mg of catalyst and 50 mL of cyclohexane as solvent in a 200 mL
stainless steel batch reactor . The analysis of reactants and products was followed by a GC-MS device
(Shimadzu GCMS-QP5050), using a chiral β-Dex 225, 30 m column (Supelco) and helium as
carrier gas.
4. Conclusions
The results obtained in the hydrogenation of 1-phenyl-1,2-propanodione on Pt/CNTsZrO2 catalysts
show that the amount of chiral modifier present on the metallic sites significantly affects catalytic
activity, enantioselectivity and the asymmetric hydrogenation reaction. The catalysts were completely
regioselective. Thus, only the hydrogenation of the C=O bond located close to the aromatic ring took
place, and preferentially led to the corresponding R-hydroxyketone.
Acknowledgements
The authors thank FONDECYT Grant No. 1061001 for financial support
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