Silica supported rhodium metal nanoparticles stabilized with (-)-DIOP. Effect of ligand concentration and metal loading on the enantioselective hydrogenation of ketones

Article abstract of DOI:10.4067/S0717-97072012000400013

Supported nanoparticles (NPs) in presence of chiral ligand (L) were synthesized for their use in enantioselective hydrogenation reactions. Catalysts were obtained by chemical reduction from rhodium chloride hydrate, RhCl ₃×3H₂O, in presence of (-)-DIOP ligand ((4R,5R)-4,5-Bis(diphenylphosphino-methyl)- 2,2-dimethyl-1,3-dioxolane) that allows to control NPs growing and to obtain solids having chiral surfaces. Chirally stabilized rhodium NPs on SiO₂ were characterized using techniques such as: TEM, electron diffraction, EDS, nitrogen adsorption-desorption isotherms and XPS. This work includes the study of some variables such as metal loading and ligand concentration and their effect in metal core sizes, catalytic activity and enantioselectivity. Catalysts properties have also been evaluated in the hydrogenation of substrates: acetophenone (AP), 1-phenyl-1,2-propanedione (PPD), 3,4-hexanedione (HD), 2,3-butanedione (BD) and ethyl pyruvate (EP) as reaction test. Ligand plays a fundamental role in the synthesis of NPs and enantioselectivity in hydrogenations reactions. That is, due to it generates metal particle size < 5.8 nm compared with unstabilized systems that generate average diameter around 14 nm. Results indicate increased activity in catalytic systems obtained from the stabilization of NPs. Enantioselectivity levels reach values up to 53% due to the chiral ligand is on the catalysts surface.

Full text of DOI:10.4067/S0717-97072012000400013

J. Chil. Chem. Soc., 57, Nº 4 (2012)
SILICA SUPPORTED RHODIUM METAL NANOPARTICLES STABILIZED WITH (-)-DIOP. EFFECT OF
LIGAND CONCENTRATION AND METAL LOADING ON THE ENANTIOSELECTIVE
HYDROGENATION OF KETONES
DORIS RUIZ1*, CLAUDIO MELLA1, JOSÉ L.G. FIERRO2 AND PATRICIO REYES1
1Universidad de Concepción, Facultad Ciencias Químicas, Edmundo Larenas 129, Concepción, Chile.
2Instituto de Catálisis y Petroleoquímica, CSIC, Marie Curie 2, Cantoblanco 28049 Madrid, Spain.
(Received: July 12, 2012 - Accepted: July 23, 2012)
ABSTRACT
Supported nanoparticles (NPs) in presence of chiral ligand (L) were synthesized for their use in enantioselective hydrogenation reactions. Catalysts were
obtained by chemical reduction from rhodium chloride hydrate, RhCl ×3H2O, in presence of (-)-DIOP ligand ((4R,5R)-4,5-Bis(diphenylphosphino-methyl)-
2,2-dimethyl-1,3-dioxolane) that allows to control NPs growing and t3o obtain solids having chiral surfaces. Chirally stabilized rhodium NPs on SiO2 were
characterized using techniques such as: TEM, electron diffraction, EDS, nitrogen adsorption-desorption isotherms and XPS.
This work includes the study of some variables such as metal loading and ligand concentration and their effect in metal core sizes, catalytic activity and
enantioselectivity. Catalysts properties have also been evaluated in the hydrogenation of substrates: acetophenone (AP), 1-phenyl-1,2-propanedione (PPD),
3,4-hexanedione (HD), 2,3-butanedione (BD) and ethyl pyruvate (EP) as reaction test.
Ligand plays a fundamental role in the synthesis of NPs and enantioselectivity in hydrogenations reactions. That is, due to it generates metal particle size <
5.8 nm compared with unstabilized systems that generate average diameter around 14 nm. Results indicate increased activity in catalytic systems obtained from
the stabilization of NPs. Enantioselectivity levels reach values up to 53% due to the chiral ligand is on the catalysts surface.
Keywords: chiral ligand, (-)-DIOP, enantioselective hydrogenation, Rh/SiO2, stabilized nanoparticles.
INTRODUCTION
EXPERIMENTAL
The asymmetric synthesis has gained importance since 2001 when it was
awarded the Nobel Prize in Chemistry to Knowles, Noyori and Sharples by
scientific contributions in the field of asymmetric homogeneous catalysis1.
Since then, and due to a growing demand for enantiomerically pure products for
industry, there has been an attempt to homogeneous asymmetric hydrogenation
of substrates for the production of chiral products as herbicides, insecticides,
General
Rhodium NPs catalysts stabilized with (-)-DIOP on SiO2 were synthesized
in inert atmosphere of argon at pH= 8 to prevent the decomposition of
phosphorus ligand (-)-DIOP (see Fig. 1a).
Prochiral substrates (PPD, BD, EP, HD and AP in Fig.1 b-f) and solvents
used in enantioselective hydrogenations were previously purified by distillation.
(-)-DIOP was used as received.
food, fragrances and especially drugs2,3
.
Even though asymmetric catalysis has been studied in homogeneous,
heterogeneous and enzymatic phase, currently most of the processes to get
chiral products are conducted by homogeneous processes, because the catalysts
have shown to be extremely efficient and selective. Nowadays important efforts
have focused toward the “heterogenization” of the homogeneous asymmetric
catalysis or to develop strategies to immobilize chiral NPs on solid supports
to achieve simpler and more efficient catalysts able to be used in consecutive
Rhodium trichloride hydrate (RhCl3×3H2O), methyl tetraoctylammonium
(TOAB), (-)-DIOP and substrates such as EP, PPD, BD and HD were supplied
by Aldrich. Chloroform, cyclohexane and sodium borohydride (NaBH4) by
Merck, AP for Fluka and H2 (99.995%) was supplied by AGA.
cycles and ideally in continuous processes4-6
.
Catalysts used in fine chemical reactions are mainly noble metals on
supports such as SiO , Al O3, among others. In recent years, the interest in
the catalytic properti2es o2f metal colloids has increased, due to the high
metallic surface of NPs includes a large number of metal atoms available
to the substrate7. NPs of different metals have been assayed as catalysts in
hydrogenation reactions of aromatic hydrocarbons8-10 and recently it has been
extended to enantioselective hydrogenation reactions11-16, hydroformylation
and allylic alkylation17-21
.
Usually traditional synthesis generates colloids diameter higher than 10
nm. Recently it has been developed stabilization processes that generate NPs
diameters from 1 to 10 nm. Small diameters and high reproducibility in the
formation of metallic particles are related with the addition of stabilizers, which
interacts with metal during its reduction to generate particular properties in the
material, i.e. chiral surface in NPs with specific shape and composition22-29
.
Figure 1. Chiral ligand a) (-)-DIOP and prochiral substrates b) PPD,
c) BD, d) EP, e) HD and f) AP used in NPs synthesis and enantioselective
hydrogenations respectively.
The aim of the present work is to study the synthesis of colloids by a
method for stabilizing NPs30-32. Stabilization is carried out with a chiral agent in
different amounts to obtain asymmetric surface and thus to generate particles of
small metal diameter. Catalysts with different ligand concentration and metal
loading have been used as catalysts in enantioselective hydrogenation reactions
of AP, PPD, HD, BD and EP as test reaction. These substrates are interesting
due to the applications that have some of its (R)-product of hydrogenation33-35
in synthesis of chiral drugs. Thus, (R)-1-hydroxy-1-phenylpropan-2-one,
product of PPD hydrogenation, is a precursor for the production of ephedrine
and analogues used as a nasal and bronchial decongestant among others36.
Catalysts synthesis
Stabilized NPs have been synthesized at room temperature for addition
of RhCl3×3H2O in 2 mL of distilled H O. Rh3+ dissolution forms a biphasic
system with 2 mL of TOAB dissolved2in 2 mL of chloroform. After 1 h of
stirring in an inert atmosphere (-)-DIOP dissolved in 25 mL of CH2Cl2 was
added. Prior to incorporation of ligand the pH of the solution was adjusted to
alkaline (pH= 8.0) with a saturated solution of NaHCO3 (100 g×L-1), to avoid
e-mail: doruiz@udec.cl.
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J. Chil. Chem. Soc., 57, Nº 4 (2012)
the decomposition of (-)-DIOP. After 30 min under stirring NaBH4, as metal
reducer, was added to the Rh3+-L system and stirred for 3 h in argon at 298 K.
The amount of TOAB and NaBH4 was 0.067 and 0.922 mmol respectively and
in the study of the effect of ligand concentration, added to NPs synthesis, the
amount of (-)-DIOP (on 1.0 wt% Rh) was varied from 0.05 to 0.20 mmol and
metal loading was studied in the range of 0.5 to 1.5 wt% Rh (L= 0.15 mmol)
following procedures reported in literature31-32. The NPs-L obtained in all cases
were washed (3 × 25 mL H2O), filtered under vacuum and stored in an inert
atmosphere.
The catalysts were labeled as: 0.5Rh-3L to 1.5Rh-3L according to weigh
percent of metal and 1.0Rh-1L to 1.0Rh-4L in relation to ligand concentration
added on NPs synthesis.
A stabilized dispersion of NPs in appropriate amount, according to wt% of
metal expected on the support, was deposited by impregnation on SiO2 (BASF,
147 m2×g-1) at 40 °C. Then, the solvent was removed under vacuum.
225 (Supelco) of 30 m long x 0.25 mm diameter with helium as carrier gas.
Products and reactants were determined by comparison of the retention time
with the pure commercial samples.
Enantioselective hydrogenations reactions and recycling
Enantioselective hydrogenation reactions were carried out in a semibatch
reactor stainless steel coated with Teflon in 50 mL of cyclohexane, 40 bar of
H2 pressure, 298 K, stirring speed of 800 rpm and molar ratio [substrate/metal]
on the range of 60 to 200 approximately. The experimental conditions were
selected to avoid transfer limitations.
A study of recycling in enantioselective hydrogenation of PPD was
performed in the same catalytic reaction under conditions previously described.
The catalysts were separated by filtration of the prior reaction, washed
repeatedly three times with 20 mL of pentane and dried by solvent extraction.
Recovery of catalysts was carried out in argon atmosphere.
Substrate, solvent and recovered catalysts were added to the batch reactor
and carried out similarly to that described previously.
Catalysts characterization
X-ray photoelectron spectroscopy (XPS) studies were performed in a
Fisons Escalab 200R spectrometer with a Mg Kα X-ray radiation (hν= 1253.6
eV) operated at 10 mA and 12 kV. Prior analysis, the samples were pretreated
in hydrogen at room temperature.
RESULTS AND DISCUSSION
Catalysts characterization
Metal particle size was determined by transmission electron microscopy
Analysis of N2 adsorption-desorption isotherms by B.E.T and BJH method
allowed to determine specific surface and pore diameter distribution for the
catalysts studied. As shown in Table 1, the studied variable, metal content
and amount of modifier, did not affect significantly the textural properties
of catalysts. The specific surface area ranged from 128 to 147 (m2 × g-1) and
average pore diameters from 17 to 23 nm. Rh-L/SiO2 catalysts have type
IV isotherm according to the classification Brunauer, Deming, Deming and
Teller (B.D.D.T). Isotherms with this kind of hysteresis loop are characteristic
of mesoporous solids with cylindrical pore geometry for all samples. These
characteristics are appropriate for catalytic applications.
(TEM) in
a JEOL JEM-1200 model EXII. Energy-dispersive X-ray
spectroscopy (EDS), Electron Diffraction (ED) and the patterns of the samples
were obtained in the same equipment.
Nitrogen adsorption-desorption isotherms at 77 K were performed in a
Micromeritics ASAP 2010 (CHEMI).
The analysis of reagents and products was followed by chiral GC analysis.
Conversion and optical yield, expressed as enantiomeric excess, (%) ee= ([R]-
[S])/([R]+[S]) x100, were calculated from analysis in a GCMS chromatograph
SHIMADZU GC-MS-QP5050 equipped with a chiral column Beta DEX
Table 1. Characterization of Rh-L/SiO2 catalysts. Effect of: (a) metallic loading and (b) ligand concentration on particle size (d), dispersion (D), SB.E.T and
pore diameter.
(a) Metallic loading
(b) Ligand concentration
SB.E.T
(-)DIOP,
mmol
SB.E.T
dpore
(nm)
%Rh
Catalyst
dTEM (nm)
D
dpore (nm)
Catalyst
dTEM (nm)
D
(m2×g-1)
(m2×g-1)
0.05
0.10
0.15
0.20
1.0Rh-1L
1.0Rh-2L
1.0Rh-3L
1.0Rh-4L
5.8
5.1
4.8
3.5
0.16
0.18
0.19
0.26
143
135
140
131
19
22
18
23
0.5
1.0
1.5
0.5Rh-3L
1.0Rh-3L
1.5Rh-3L
3.9
4.8
5.3
0.24
0.19
0.17
134
147
128
23
17
17
a) Effect of metal loading using 0.15 mmol (-)-DIOP (3L) and b) Effect of ligand concentration on 1.0wt% of Rh (1.0Rh) catalysts. Dispersion (D) was
calculated from dTEM and Rh metallic surface of 436 (m2×g-1 metal).
Energy Dispersive X-ray Spectroscopy and Electron Diffraction were
performed in order to verify the components present in Rh catalysts. Figure
2a shows electron diffraction and EDS spectrums for 1.0Rh-3L. In all cases
it was confirmed that the reduction method is effective and metal component
of the catalysts is mainly in the zero reduction state. The diffraction showed
a crystallographic direction with higher intensity of the plane (111) and an
interplanar spacing of 2.196 A°. Under the same conditions used for electron
diffraction, EDS analysis from counts versus energy allowed to verify the
presence of Rh in all catalysts, see figure 2b. The study was performed in the
range of 1 to 20 keV and despite the low percentage of metal in the catalysts,
rhodium generated peaks of intensity 3 and 20 keV referred to the internal
standard Cu (8-9 keV).
The effect of ligand concentration on the metal particle size was carried
out with the 1.0 wt% of Rh. As expected, the ligand plays an important role
to stabilize the crystallites formed during the reduction and therefore if the
concentration of the stabilizer increases the average metal particle size
decreases. In absence of ligand, diameters up 14 nm were obtained.
Surface studies of Rh catalysts were performed by XPS. Binding energies
(BE) have been obtained in comparison to C1s of 284.9 eV.
Rh 3d5/2 core level spectra showed the presence of Rh in two different
oxidation states. Rhº at 307.3 eV and Rhd+ at 309.9 eV, see figure 4. Rhodium
is mainly in a reduced state (see table 2), confirming that the reduction method
is effective, however, a fraction close to 30% of Rhd+ are present in most
of catalysts. This later species are important because they can contribute to
Rh particle size and metal dispersion were obtained from TEM and the
results are compiled in Table 1. The average particle size for catalysts, prepared
in presence of 0.15 mmol of (-)-DIOP as chiral modifier, increase from 3.9 to
5.3 nm according as metal loading vary from 0.5 to 1.5% (see Fig. 3), whereas
metal dispersion decreases as Rh content increases. The observed trend can be
understood taking into account that there is a strong tendency to the growth
of clusters when there are low amounts of ligand in colloidal synthesis. This
behavior is expected for the series due to the stabilizer containing phosphorus
which interacts with Rh during reduction with NaBH4, limiting the metal-metal
agglomeration.
polarize the C=O bond of the substrates making easier it hydrogenation 37-39
.
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J. Chil. Chem. Soc., 57, Nº 4 (2012)
Figure 2. a) Electron diffraction and b) EDS spectrum for 1.0Rh-3L
catalyst.
Figure 4. Binding energies (eV) of internal levels of Rh catalysts
synthesized in different chiral ligand concentrations a) 1.0Rh-1L b) 1.0Rh-2L
c) 1.0Rh-3L and d) 1.0Rh-4L.
In the series of catalysts with 0.15 mmol (-)-DIOP and different Rh content,
the Rh/Si atomic surface ratio increases with Rh content on the catalysts. The
observed enhancement in the Rh/Si atomic surface ratio in a factor of 18 as the
bulk ratio increases almost 3 times is indicative of changes in the location of
metallic crystals. Thus, the catalyst with lower Rh content generates smaller
chiral metallic crystals which remain mainly inside the pore structure of the
support. As the metal loading increases, a higher fraction of the NPs are in
external positions of the support and consequently easier detectable by XPS.
Similar trends can explain the Rh/Si atomic surface values obtained in the
series with variable ligand content. As was previously reported, an increase in
the ligand concentration lead to smaller metal NPs, thus this tiny particles can
be mainly located inside the pores structure of the support and therefore the
detected Rh/Si surface ratio also decreases (see table 2).
Figure 3. TEM images for a) 1.0Rh-4L and b) 1.0Rh-3L catalysts.
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J. Chil. Chem. Soc., 57, Nº 4 (2012)
Table 2. Binding energies of 3d5/2 and Rh/Si surface and bulk atomic
surface ratios of Rh catalysts.
Table 3. Conversion levels and enantioselectivity in the hydrogenation of
EP 0.02 mol×L-1 using 100 mg of Rh catalysts in cyclohexane at 298 K and 40
bar of H2.
Catalyst
Rh 3d5/2 (BE, eV)
(Rh/Si)s
(Rh/Si)b
k / 10-5
(min-1×gcat
1×mol)b
k
0.5Rh-3L
307.3 (100)
0.0016
0.00293
%
% eecmax
-
Catalyst
(min-1×at-
Rh×molec)b
Conversiona
307.7 (65)
309.8 (35)
1
1.0Rh-3L
1.5Rh-3L
1.0Rh-1L
1.0Rh-2L
1.0Rh-3L
1.0Rh-4L
0.0056
0.0293
0.0051
0.0048
0.0044
0.0019
0.00589
0.00888
0.00589
0.00589
0.00589
0.00589
0.5Rh-3L
1.0Rh-3L
1.5Rh-3L
1.0Rh-1L
1.0Rh-2L
1.0Rh-3L
1.0Rh-4L
82
97
98
95
91
97
99
7.9
9.0
9.3
7.4
5.8
9.0
8.9
1.63
0.93
0.64
0.76
0.60
0.93
0.92
21
26
11
19
29
26
27
307.7 (77)
309.8(23)
307.8 (75)
309.9 (25)
307.7 (78)
309.8 (22)
307.8 (67)
309.9 (33)
a % conversion at 360 min of reaction. b Pseudoconstants of first order rate
calculated up to 60 min. c% ee referred to the formation of (R)-ethyl lactate.
307.8 (73)
309.9 (27)
Figure 6 shows the main products in the enantioselective hydrogenation
of HD, BD, PPD and AP. (-)-DIOP as chiral inducer on the surface allows to
obtain only R-enantiomers. In the enantioselective hydrogenation of ketones,
catalytic activity and enantioselectivity show opposite trends. Thus, substrates
easy to hydrogenate usually display a low %ee. Furthermore, the structure
of each substrate affects the catalytic behavior, in lineal diketones prevails
activity with respect to enantioselectivity levels, opposite to aromatic ketones.
Enantioselective hydrogenations
The asymmetric hydrogenation of EP was performed as a reaction test
to evaluate the activity and enantioselectivity of all synthesized catalysts (see
Fig. 5).
It can be seen that the reaction progress continuously according a pseudo-
first order reaction. The conversion levels reached by catalyst with higher metal
loading (1.5Rh-3L) are similar to 1.0Rh-3L whereas the 0.5Rh-3L catalyst
displayed lower conversion. This may be explained in terms of availability
metal active sites which in turns depend on metal dispersion and location of
metal crystals. With regard to the enantioselectivity, in the enantioselective
hydrogenation of EP, it was found a preferential formation of (R)-ethyl lactate
(Fig. 6a).
Table 4 shows the conversion levels of different substrates studied at 420
min of reaction and ee obtained using 1.0Rh-3L catalyst. It should be mentioned
that even though the enantioselectivity levels remains approximately constant
with the conversion in the different studied reactions, conversion levels and
pseudo constant rate are higher in lineal diketones, however, are lower in
enantioselectivity, in line with results already reported33-34. On the other hand,
aromatic ketones, PPD and AP, show higher %ee and lower activity, attributed
to the structure of the aromatic compounds.
Figure 6. Main products in the enantioselective hydrogenation of a) EP, b)
HD, c) BD, d) PPD and e) AP.
Table 4. Pseudo first order rate constants, conversion levels and
enantioselectivity in the hydrogenation of EP, HD, BD, PPD and AP.
Conditions: 0.02 mol×L-1 on 100 mg of 1.0Rh-3L catalyst in cyclohexane at
298 K and 40 bar of H2.
Figure 5. Evolution of the conversion and enantioselectivity on time for EP
hydrogenation on 100 mg of Rh catalysts, 0.02 mol×L-1 of EP in cyclohexane
at 298 K and 40 bar of H2.
k / 10-5
Substrate
% Conversion a
% eecmax
(min-1×gcat-1×mol)b
The ee do not change significantly with the conversion level. In the study
of metal loading, 1.0Rh-3L catalyst displayed the highest ee value, 26%.
1.5Rh-3L catalyst compared to 1.0Rh-3L, contains an excess of 50% Rh atoms,
similar metal dispersion but a lower ligand/metal ratio, therefore a lower ee
value can be expected. On the other hand, in spite of the 0.5Rh-3L catalyst
presents the highest ligand/metal ratio, its metal dispersion is also the highest
and consequently the metal particle size is the smallest which allow to locate
them in the interior of the pore structure, and the accessibility of the substrate to
the chiral centers is more limited. Thus, this catalyst should exhibit the lowest
activity and the highest selectivity.
HD
BD
89
94
6.7
26
23
12.8
72
2.6
53
68d1
50d2
45d3
41d4
2.5d1
1.8d2
1.6d3
1.2d4
45d1
38d2
37d3
37d4
PPD
AP
88
2.3
48
a% Conversion at 420 min of reaction. b Pseudoconstants of first order rate
From the evolution of substrate concentration with time, mainly during the
first 60 min the pseudo first order rate constant was obtained and the results are
compiled in Table 3.
c
calculated up to 60 min. % ee referred to the formation of (R)-product, see
figure 6. d Results of recycling. 1, 2 and 3 correspond to the cycle number.
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J. Chil. Chem. Soc., 57, Nº 4 (2012)
One of the most interesting products in the hydrogenation of substrates
is (R)-1-hydroxy-1-phenylpropan-2-one, from PPD. This compound is
commonly known as L-PAC and its importance become because it acts as a
key intermediate of various compounds used for treatment of hypotension,
nasal decongestants and asthma40. It production was preferencial with respect
to others products of PPD hydrogenation. During the formation of a new pair of
enantiomers in minor amount, the ee values vary reached a maximum of 53%
at 420 minutes of reaction.
An unmodified catalyst, 1% Rh/SiO2, obtained in similar conditions but
without (-)-DIOP as stabilizer, was also prepared as comparison with 1.0Rh-
3L. Average particles size of this catalyst was higher as consequence of it was
prepare in absence of the stabilizer ligand and therefore the growing of the metal
particle is higher. Additionally, the lack of chiral surface leads to the racemic
mixture. These catalytic tests were performed only on the hydrogenation in
similar conditions of PPD substrate.
Figure 7 shows the reactant consumption and the formation of products
in PPD hydrogenation. It can be observed as the diketone is converted, the
hydrogenation of both carbonyl bonds may occurs, however, is favoured the
hydrogenation of carbonyl group closer to the aromatic ring. After 100 min of
reaction the slight formation of (S) and (R)-2-hydroxy-1-phenylpropan-1-one
was observed, but in lower extent.
CONCLUSIONS
The hydrogenation of prochiral substrates at 40 bar and 298 K was studied
on different Rh chiral catalysts. The obtained results indicate that catalysts
synthesized under mild conditions allow to obtain active and selective NPs
on SiO in asymmetric hydrogenation, without further treatment, because the
surface2contains chirally modified metal sites.
The variation of metal loading in the catalysts allows a slight change in
catalytic activity, and it was also found that the concentration of chiral ligand
added in the synthesis of Rh NPs significantly affects the metal size and the
enantioselectivity of each system in particular.
The reuse of catalyst shows high activity and enantioselectivity in a first
recycle decreasing slightly with increasing the amount of recycling. These
changes are explained in terms of small leaching of metal NPs.
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ACKNOWLEDGEMENTS
This work was supported by the FONDECYT project Nº 1061001. D.R
thanks Mecesup for fellowship grant.
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