Enantioselective hydrogenation of 1-phenyl-propane-1,2-dione on immobilised cinchonidine Pt/SiO2 catalysts

Article abstract of DOI:10.1016/j.molcata.2011.07.016

Chirally modified SiO₂ containing different amounts of cinchonidine was prepared by the chemical modification of cinchonidine (CD) with trimethoxysilane over SiO₂ that had been chemically activated with 1,4-dioxane/HCl. This solid wa

Full text of DOI:10.1016/j.molcata.2011.07.016

Journal of Molecular Catalysis A: Chemical 348 (2011) 30–41
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Journal of Molecular Catalysis A: Chemical
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Enantioselective hydrogenation of 1-phenyl-propane-1,2-dione on immobilised
cinchonidine Pt/SiO2 catalysts
Cristian H. Camposa,∗, Marcelo Oportusa, Cecilia Torresa, Claudia Urbinaa, José L.G. Fierrob,
Patricio Reyesa,∗
a Edmundo Larenas 129/Universidad de Concepción, Departamento de Fisicoquímica, Laboratorio de Catálisis por Metales, Concepción, Chile
b Grupo de Energía y Quimica Sostenible, Instituto de Catálisis y Petroleoquímica, CSIC, C/Marie 2, Cantoblanco, 28049 Madrid, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 May 2011
Received in revised form 24 July 2011
Accepted 26 July 2011
Available online 10 August 2011
Chirally modified SiO2 containing different amounts of cinchonidine was prepared by the chemical mod-
ification of cinchonidine (CD) with trimethoxysilane over SiO2 that had been chemically activated with
1,4-dioxane/HCl. This solid was used to support Pt catalysts containing 1 wt%Pt obtained by the chemical
reduction of hexachloroplatinic acid with H2 at 298 K and 40 bar. The materials were characterised by ele-
mental analyses of C, H, and N, as well as thermal gravimetry (TG), DRIFT, 13C NMR, 29Si solid state NMR,
N2 adsorption–desorption at 77 K, X-ray diffraction (XDR), XPS, and HR-TEM. Catalytic activity for the
hydrogenation of 1-phenyl-propane-1,2-dione was evaluated in a batch reactor at 298 K and 40 bar, and
recycling of the catalysts yielding the largest enantiomeric excess in the products were also studied. All
catalysts were found to be active in the reaction, with enantiomeric excesses of the target product (1-R-
phenyl-1-hydroxy-2-propanone) ranging from 35 to 50%. The best catalyst studied was SiO2-supported,
with a nominal content of 7.5 mmolCD/L g. The recycling tests showed a loss of activity that was attributed
to the surface modification of the catalyst.
Keywords:
Heterogenisation
Platinum
Cinchonidine
1-Phenyl-propane-1,2-dione
Heterogeneous catalyst
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
inorganic supports through the immobilisation of organometallic
catalysts on SiO2 and its derivatives [9]. Jamis et al. [10] described
Catalytic enantioselective hydrogenation is one way to obtain
enantiopure compounds from reducible moieties such as C C, C N,
and C O. For heterogeneous catalysis, supported noble metals that
operate in the presence of a chiral auxiliary molecule are often used.
The chiral auxiliary is generally a derivative of quinine, an alkaloid
that presents three characteristic groups in its structure: a quino-
line ring; an asymmetrical, bridge-type centre that incorporates a
hydroxyl group; and a quinuclidine ring that can have a vinyl group
(–CH CH2) on one of its vertices [1]. This inducer is generally added
to the reaction system in situ and has been widely studied in hydro-
genation reactions of ␣-keto esters and ␣-diketones [2–5]. Diverse
metals such as Pt, Ir, Pd, Rh, and Ru supported on SiO2, Al2O3, and
others, have been used as an active phase. Cinchonidine (CD) has
been the main chiral auxiliary used in hydrogenation studies of liq-
uid phase ethyl pyruvate and 1-phenyl-propane-1,2-dione (PPD)
[3,5–8].
the encapsulation via sol–gel of organometallic complexes of Ru
to be used in aqueous phase hydrogenations. Fan et al. proposed
the use of insoluble polymers as supports for homogeneous cat-
alysts immobilised by entrapment [11]. Complexes can also be
supported through anchoring mediated by a coupling agent, typi-
cally trimethoxysilane derivatives functionalised with Cl, NH2, or
SH on materials such as Al2O3, La2O3, MCM-41, ZrO2, TiO2, or Fe2O3,
among others [12–16].
The CD molecule, like organometallic complexes, can be
anchored on the surface of a solid. Several authors have reported
the anchoring of quinine derivatives on SiO2 and other supports
used as stationary phases in HPLC. Lämmerhofer et al. reported
anchoring mediated by carbamates with the OH group at the C9
position [17], whereas Ma et al. used sulphur bridge-type bonds
to immobilise quinines through the vinyl group of the quinucli-
dine ring [18]. Pesek et al. [21,22] studied the surface modification
of SiO2 with organic molecules, finding alkoxysilanes to be more
versatile “coupling agents” (CA) for immobilising this type of sub-
strate. The alkoxysilane was anchored by transesterification of the
alkoxidegroups, makingthe prior activationof the SiO2 surfacenec-
essary due to the low quantity of OH groups found on the oxides
typically used for catalysis and the ensuing low surface reactivity in
this type of reaction [23–28]. The immobilisation of CD by means of
A diverse range of improvements have been reported in the
preparation of heterogeneous catalysts, including modifications of
∗
Corresponding authors.
E-mail addresses: ccampos@udec.cl (C.H. Campos), preyes@udec.cl (P. Reyes).
1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.07.016

C.H. Campos et al. / Journal of Molecular Catalysis A: Chemical 348 (2011) 30–41
31
AMX-300 spectrometer (300 MHz for 1H, 75 MHz for 13C) using
trimethylsilane as a standard; all the results obtained from the NMR
of 1H and 13C were compared with the results from Toukoniitty
et al. [19,20] with respect to the observed allocations and dislodged
chemical signals. Solid-state 13C and 29Si CP NMR spectra were
recorded at 100.6 MHz and 79.49 MHz, respectively, using a Bruker
AV 400 WB spectrometer. Diffuse reflectance infrared Fourier
transform (DRIFT) spectra were obtained using a JASCO FT/IR-6300
spectrometer. XRD patterns were recorded on a RigakuD/max-
2500 diffractometer with Cu K␣ radiation at 40 kV and 100 mA.
N2 adsorption-desorption analysis was performed at 77 K on a
Micromeritics ASAP 2010 apparatus. Specific surface areas were
determined via the BET (Brunauer–Emmett–Teller) equation, using
adsorption data in the relative pressure range of 0.05–0.3, and pore-
size distributions were estimated using the BJH model. HR-TEM
micrographs were obtained with a HR-TEM Philips CM-200 system.
Photoelectron spectra (XPS) were recorded using an Escalab 200 R
spectrometer equipped with a hemispherical analyser and using
non-monochromatic Mg K␣ X-ray radiation (hꢀ = 1253.6 eV). The
surface Pt/Si and N/Si atomic ratios were estimated from the inte-
grated intensities of Pt 4f, Si 2p, C 1s, and N 1s lines after background
subtraction and correction by the atomic sensitivity factors [41].
The spectra were fitted to a combination of Gaussian–Lorentzian
lines of variable proportions. The binding energy of the Si 2p peak
at 103.4 eV was taken as an internal standard.
the reaction between the CA and the anchored inducer have been
reported [29,30] and in these studies it has been demonstrated that
the presence of remaining groups of the CA on the support surface
interfere in the catalytic performance [30]. If the chiral inducer is
modified prior its immobilisation on the support surface it would
allow a better control on the surface organic species, and therefore
it is likely to assess that the anchored species correspond only to
chiral inducer.
Modifications of CD were studied by the group of Toukoniitty
[19,20], who introduced triethoxysilane groups into the vinyl group
of the quinuclidine heterocycle and made multiple modifications
to the hydroxyl group at C9 using various derivatives of chlorosi-
lanes in enantioselective hydrogenation studies of PPD. According
to those authors, modifications in the hydroxyl group heavily alter
the capacity for enantioselectivity on the part of the chiral auxiliary.
Thus, the inducer of chirality can be anchored on a supported metal-
lic catalyst rather than adsorbed onto the active phase, potentially
giving rise to systems reusable for consecutive batch cycles.
The presence of organic content on the surface of a support
changes its thermal stability [31]. Therefore, catalysts with an
immobilised organic molecule on the support must be prepared
for surface metal deposition using an alternative to the traditional
synthesis of supported metal catalysts (precursor impregnation,
calcination, and reduction between 200 and 500 ◦C). Platinum has
given the best results for both activity and selectivity in the hydro-
genation of prochiral substrates and has been widely studied by
Toukoniitty et al. [6,7,19,20,32,33]. Different physical and chemi-
cal methods have been used to prepare nanoscale Pt [34], including
chemical reduction and stabilisation with polymers and oligomers
[35] and electrostatic stabilisation [36], among others. For the syn-
thesis of Pt colloids, Bönnemann and Braun [37] used formic acid
as a reducing agent at 100 ◦C in the presence of CD as a stabilising
agent. Roucoux et al. [38] assayed the activity of colloidal phases
stabilised with surfactants under high pressure H2 in the hydro-
genation of ␣-keto esters. This methodology was used by Reyes
et al. to support colloids on SiO2, generating catalysts employed
in the hydrogenation of ethyl pyruvate and PPD [39,40]. These
authors found somewhat lower enantioselectivity in stabilised sys-
tems than that obtained when using an inducer adsorbed on the
catalyst surface.
2.2. Activation of SiO2
The surface activation of SiO2 was performed in a round-bottom
flask in which 3 g of SiO2 was mixed with 24 mL of 1,4-dioxane and
3 mL HCl (3.1 mol/L). The mixture was stirred for 30 min at 353 K,
filtered, and dried under vacuum for 4 h at 393 K.
2.3. Preparation of modified CD
The modification of CD and its subsequent hydrosilation were
performed according to methods reported by Toukoniitty and co-
workers [19]. An ice-cooled solution of CD (2.5 g, 8.5 mmol) in
THF containing TEA (1.2 mL, 8.5 mmol) was added dropwise to
TMCS (1.0 mL, 8.5 mmol). The reaction mixture was stirred for 20 h
at room temperature and then for 2 h at 60 ◦C. The product was
extracted with chloroform (50 mL) and washed with water (3×
50 mL). The water layer was extracted with additional chloroform
(50 mL) and the combined organic extracts were dried over sodium
sulphate. Evaporation of the solvents left 2.5 g of solid (label CD-9-
O) (see Scheme 1). 1H NMR (CDCl3, ı): 8.93 (d, 1H, H-2ꢀ), 8.19 (dd,
1H, H-8ꢀ), 7.81 (br, 2H, H-5ꢀ, H-7ꢀ), 7.55 (ddd, 1H, H-6ꢀ), 7.30 (br,
1H, H-3ꢀ), 5.62 (ur, 2H, H-9, H-10), 5.07 (ur, 2H, H-11), 3.31 (ur, 1H,
H-6b), 3.15 (ur, 2H, H-2a, H-8), 2.68 (ur, 1H, H-7b), 2.43 (m, 1H,
H-6a), 2.11 (ur, 2H, H-2b, H-3) 1.89 (m, 1H, H-4), 1.49 (m, 1H, H-
Our review of the literature has led us to propose the synthesis of
catalysts via immobilisation of CD on the SiO2 surface and dispersal
of Pt over this modified support as an active phase obtained by the
reduction of an inorganic precursor under high pressure H2 during
the hydrogenation of PPD under predetermined conditions.
2. Experimental
2.1. General
5b), 1.29 (m, 1H, H-5a), 0.85 (m, 1H, H-7a), 0.18 (s, 9H, Si–CH3). 13
C
NMR (CDCl3, ı): 149.59 (C-2ꢀ), 148.38 (C-8aꢀ), 146.20 (C-4ꢀ), 138.00
(C-10), 130.23 (C-8ꢀ), 129.72 (C-7ꢀ), 128.10 (C-6ꢀ), 124.72 (C-4aꢀ),
123.52 (br, C-5ꢀ), 118.61 (br, C-3ꢀ), 116.84 (C-11), 68.74 (br, C-9),
60.84 (C-8), 56.70 (C-2), 43.06 (br, C-6), 35.49 (C-3), 29,69 (C-4),
27.12 (C-5), 18.62 (br, C-7), 0.35 (3C, Si–CH3).
All air-sensitive reactions were performed in a Schlenk flask
using an inert argon (Ar) atmosphere. Tetrahydrofuran (THF,
Merck) was dried over metallic sodium/benzophenone; toluene
(99%, Merck) was dried over metallic sodium; and triethylamine
(TEA) was distilled at reduced pressure prior to use. CD (96%,
Aldrich), trimethylchlorosilane (TMCS 98%, Merck), trimethoxysi-
lane (TMS 98%, Merck), platinum cyclooctadienyl (II) chloride
(PtCODCl2, Aldrich), anhydrous sodium sulphate (Na2SO4, Merck),
SiO2 (Grace Davison), H2PtCl6·6H2O (40%Pt, Merck), NaOH (Merck),
and other solvents (Merck) were used as received.
2.4. Hydrosilation of modified CD
PtCODCl2 (0.018 g, 0.049 mmol) and TMS (0.77 mL, 6.0 mmol)
were added at 313 K to a solution of CD-9-O (2.0 g prepared
according to the previous reaction, approx. 5.5 mmol) in toluene
(20 mL). The reaction mixture was stirred for 5 h at 363 K
under a N2 atmosphere. Purification by flash chromatography
(hexane–acetone–TEA, 40:18:1) gave 1.25 g of the desired product
as a yellowish amorphous material (TMS-CD-9-O). 1H NMR (CDCl3,
Elemental analyses of C, H, and N were performed on a LECO
CHNS-932 analyser. TG studies were performed with a Mettler
Toledo Thermogravimetric TGA/SDTA 851 using an O2 flow of
25 mL/min and a temperature ramp of 1 K/min from 298 to 1000 K.
1
NMR spectra for 1H and 13C{ H} were obtained on a Bruker

32
C.H. Campos et al. / Journal of Molecular Catalysis A: Chemical 348 (2011) 30–41
Scheme 1. Synthetic route for CD modification.
ı): 8.88 (d, 1H, H-2ꢀ), 8.14 (dd, H-8ꢀ), 7.76 (br, 2H, H-5ꢀ, H-7ꢀ), 7.50
(ddd, 1H, H-6ꢀ), 7.26 (br, 1H, H-3ꢀ), 5.61 (br, 1H, H-9), 3.95 (q, 6H,
J = 7.0 Hz, SiOCH3), 3.39 (ur, 1H, H-6b), 3.03 (ur, 2H, 1H, H-8), 2.66
(ddd, H-6a), 2.21 (ur, 1H, H-2b), 2.03–1.78 (ur, 3H, H-4, H-5b, H-
7b), 1.41 (ur, 1H, H-3), 1.24 (m, 1H, H-10), 1.24–1.16 (ur, 2H, H-5a,
H-7a), 1.22 (m, 2H, H-11), 0.80 (s, 9H, Si–CH3). 13C NMR (CDCl3, ı):
149.63 (C-2ꢀ), 148.40 (C-4ꢀ or C-8aꢀ), 146.05 (C-4ꢀ or C-8aꢀ), 130.28
(C-8ꢀ), 129.77 (C-7ꢀ), 128.23 (C-6ꢀ), 124.69 (C-4aꢀ), 123.77 (br, C-5ꢀ),
118.59 (br, C-3ꢀ), 68.60 (br, C-9), 60.98 (C-8), 60.69 (C-2), 56.85 (3C,
SiOCH3), 43.03 (br, C-6), 35.67 (C-3), 27.32 (C-5), 27.02 (C-10), 25.29
(C-4), 24.77 (br, C-7), 11.49 (C-11), 0.34 (3C, Si–CH3).
consecutively with chloroform to clean the surface and then dried
at 373 K for 24 h before reuse.
3. Results and discussion
3.1. Synthesis and characterisation of the precursor and supports
The synthetic route for CD modification utilises a silane ether
at the hydroxyl group of carbon 9 in order to protect the stere-
ogenic centre responsible for enantioselectivity, as reported by
other authors [1,19,32]. The PtCODCl2 catalyst is known to be a good
alternative to Karstedt’s catalyst and to H2PtCl6, the two most stud-
ied catalysts, and the use of TMS-CD-9-O resulted in hydrosilation
yields of around 60% due to the addition of TMS. Several authors
[31,42,43] have shown that activation of the vinyl group can be
achieved using both of the carbons of the C C bond, forming an ␣-
adduct when the addition is by a methylene carbon or a ␤-adduct if
by addition of a methylene carbon. In the present study, the catalyst
was 100% selective in the addition of CD at C11 (␣-adduct).
2.5. Synthesis of modified SiO2
Five supports were prepared with different amounts of CD (0.5,
2.5, 5.0, 7.5, or 10 mmol/L gsupport), simulating the CD fractions that
are added in traditional systems (range: 0.5–10 mmol/L gsupport).
Two grams of activated SiO2 and the necessary quantity of TMS-
CD-9-O dissolved in toluene were placed in a round-bottom flask,
and the volume of the system was raised to 50 mL with toluene.
The reaction was refluxed for 12 h and subsequently filtered and
washed two times with 40 mL toluene and 20 mL chloroform. The
solid was dried in a furnace for 1 h at 353 K. The solid obtained
was treated by refluxing it in a mixture of methanol/THF for 20 h
and was then washed with 100 mL of n-pentane. Finally, the solids
were dried under vacuum for 4 h at 120 ◦C. All the samples of mod-
ified supports were tabulated as SiCD(x), where x is the nominal
concentration (10−3 mmol CD/g L).
SiO2 activation was performed primarily to enrich the surface
in the Si–OH groups that are responsible for anchoring TMS-CD-
9-O, as reported by Pesek et al. [21,22]. Once anchored on the
surface of the solid, the C9 hydroxyl group was deprotected. This
was achieved by refluxing the solid with methanol for 24 h, dis-
placing the trimethylsilane group to recover the OH group. Table 1
summarises the elemental analyses of the supports. The total con-
tent of CD anchored on the surface was lower than the nominal
content, and the decrease in anchoring became more pronounced
as the nominal content of the modifier increased. This could be
explained by diffusion phenomena in the anchoring reactions, as
increasing the degree of coverage by TMS-CD may hinder access
to the methoxy groups of the coupling agent at the SiO2 surface
due to the steric impediment of the quinuclidine and quinoline
rings belonging to the previously anchored molecules and to other
steric conflicts generated during the deprotection treatment. As
indicated by Blümel and other authors [44–46], species of the type
(CH3)3–SiOCH3 were formed that were able to exchange with the
surface silanol groups as well as the methanol. Thus, we based our
calculations on N (%), the only true indicator of TMS-CD content
anchored on SiO2. The anchoring of impurities by the deprotec-
tion treatment, in addition to the contribution by the exchange
phenomenon, was one of the reasons for the high C (%) and H (%)
values found in the samples relative to the nominal C/N (see Fig. 1).
Fig. 1 shows the distribution of the atomic ratios determined by
elemental analysis (at.%). At low TMS-CD contents, organic matter
was enriched and as the surface content increased, the C/N ratio
declined, falling below stoichiometric values and reaching nomi-
nal values above 5.0 mmol CD/L g. The H/N ratio was always greater
than the calculated value due to the surface activation.
2.6. Catalyst preparation
The catalyst (1 g) was prepared at 1%Pt mass using the
five modified supports and activated SiO2. These are denoted
1%Pt/TMS-CD(x), where x: 0–10. The methodology is analogous to
that reported by Roucoux et al. [38]. The support plus the nec-
essary quantity of H2PtCl6·6H2O and a stoichiometric quantity of
NaOH (0.5 mol/L) were added to a batch reactor equipped with a
Teflon cup. The pressure was adjusted to 40 bar H2 and the mixture
was reacted for 2 h with constant magnetic stirring to obtain the
reduced supported metal. The solid was filtered and washed to a
constant pH and conductivity. Finally, it was dried in a furnace at
373 K for 1 h.
2.7. Catalytic activity
The catalytic assays for PPD hydrogenation were performed
in a stainless steel Parr-type batch reactor at a concentration of
0.01 mol/L of substrate using cyclohexane as a solvent and stir-
ring at 600 rpm under 40 bar H2 pressure. Analyses of reactants and
products were followed by gas chromatography mass spectrometry
using a GC–MS instrument (Shimadzu GCMS-QP5050) with chiral
␤-Dex 225, a 30-m column (Supelco), and helium as the carrier gas.
The recycling assays were performed by filtering the catalyst from
the reaction medium. The filtered catalyst was washed three times
To explain the apparent loss of C (%), we studied the TG of the
precursor and the supports in an oxidising atmosphere, simulat-
ing the process of elemental analysis. Fig. 2 shows the curves of
weight loss in percentage of mass as a function of the tempera-
ture for TMS-CD and the various synthesized supports. It should be
noted that the species anchored on the support was TMS-CD-9-O,

C.H. Campos et al. / Journal of Molecular Catalysis A: Chemical 348 (2011) 30–41
33
Table 1
Elemental analysis of supports, catalysts, and cycles studied in the enantioselective hydrogenation of PPD.
Sample
C (%)
H (%)
N (%)
Concentration (10−4 mol/L g)
Yield (%)
1.73
0.52
0.06
0.43
85.7
SiCD(0.5)
1.02a
0.40a
0.06a
0.42a
82.1a
3.87
0.70
0.29
2.0
81.4
SiCD(2.5)
SiCD(5.0)
SiCD(7.5)
2.12a
0.62a
0.26a
1.8a
72.9a
5.00
0.94
0.53
3.8
75.0
3.05a
0.70a
0.38a
2.7a
54.3a
5.50
1.06
0.69
5.0
65.2
4.01a
0.82a
0.51a
3.6a
48.1a
3.97b,1
4.83b,2
5.63b,3
7.04b,4
9.06b,5
0.85b,1
0.94b,2
1.01b,3
1.17b,4
1.30b,5
0.45b,1
0.42b,2
0.41b,3
0.41b,4
0.40b,5
3.2b,1
3.0b,2
2.9b,3
2.9b,4
2.8b,5
89.1b,1
82.2b,2
80.2b,3
80.2b,4
79.2b,5
5.76
1.10
0.72
5.1
51.4
SiCD(10)
4.29a
0.85a
0.55a
3.9a
38.9a
a
Catalysts at 1 wt%Pt.
,xReused catalysts, where x corresponds to the run number and 5 is the catalyst recovered from the last cycle.
b
and the TG of the TMS-CD species obtained with the TMS-CD-9-
O treatment was performed under the same conditions as that of
the synthesized supports. The curve obtained for the TMS-CD pre-
cursor showed thermal stability to approximately 473 K, at which
point the precursor began to lose mass, passing through an inter-
mediate stage (620–670 K) in which it was assumed to have lost
most of the aliphatic fraction. The aromatic fraction began to react
at higher temperatures, forming intermediate species of the type
SiCxOy, leaving approximately 18% at 770 K that was highly stable
at temperatures greater than 1270 K, as reported by Schiavon et al.
[47]. In relation to the supports, we noted that SiO2 had a low quan-
tity of surface OH groups and once the support was activated, mass
was lost due to an increase in the surface concentration of Si–OH.
A drop in the curves of the heterogenised supports with TMS-CD
was also found and attributed to the loss of organic matter from
the TMS-CD functionalities, analogous to that found for the pure
precursor, exhibiting initial decomposition temperatures to values
close to 523 K in the case of SiCD(10).
Fig. 3 shows the DRIFT spectra of the synthesized supports.
In the case of SiCD(0.0), we observed an intense band around
3500–4000 cm−1 corresponding to the stretching vibration mode of
the hydroxyl groups of activated SiO2. As the TMS-CD was anchored
to the solid, the band decreased and vibrations corresponding to
C–H bonds appeared at frequencies of 3000–2900 cm−1. These
bands increased in intensity along with the content of organic
matter on the solid. A distinct band appeared between 1400 and
1500 cm−1 corresponding to vibrations of Si–CH3 bonds. This band
was attributed to the trimethylsilane species (impurities) anchored
on the surface of the support, as verified by NMR of the solids
(see Fig. 4). Fig. 4(a) shows an unusual band of chemical shifts
between −1.0 and −2.0 ppm corresponding to Si–CH3 species in
Fig. 1. Distribution of atomic ratios determined by percentage of atoms detected
by elemental analysis on the supports and synthesized catalysts by anchoring of
TMS-CD. (a) C/N and (b) H/C.
Fig. 2. TG curves for the synthesis supports. Conditions: O2 atmosphere to
25 mL/min flow and temperatures 298–1000 K.

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C.H. Campos et al. / Journal of Molecular Catalysis A: Chemical 348 (2011) 30–41
Fig. 3. DRIFT of the supports modified with TMS-CD.
all the samples and a significant increase of characteristic bands
of the aromatic and aliphatic region corresponding to TMS-CD as
the loading increased. Fig. 4(b) shows the characteristic signal of
SiO2 corresponding to an atom of Si tetracoordinated by atoms of
O from the network (−100 ppm) and split due to the activation
of the support. As the TMS-CD content increased on the surface,
this splitting became less significant and, as reported by Yu et al.
[46], the characteristic signals of anchoring of the types T2 and
T3 at shifts between −50 and −80 ppm began to appear. A sig-
nal between 13 and 14 ppm, typical of CH3–Si–O–Si species, was
found for all the supports. This finding, when taken together with
the DRIFT and 13C NMR data, is indicative of the presence of impuri-
ties (trimethylsilane derivatives) that reacted with surface silanols
in the deprotection treatment.
Analysis of N2 adsorption–desorption isotherms and SBET
showed that all the solids were mesoporous and had type IV hys-
teresis cycles, corresponding to cylindrical pores (Fig. 5). Moreover,
all samples presented monomodal pore-size distributions. Table 2
shows the surface areas and average pore diameters of the pre-
pared solids. This information allows us to infer that as the TMS-CD
content increased on the supports, the values for surface area, pore
volume, and pore size decreased due to the anchoring of the organic
molecule on the surface of the activated SiO2.
Fig. 5. N2 adsorption–desorption isotherms of 77 K on Pt/SiCD(x) catalysts.
the chiral auxiliary in open conformation 3, which is the optimum
for enantioselectivity, as reported by Baiker et al. [2]. The stoi-
chiometric addition of NaOH to the system allowed us to obtain
a neutral medium at the end of the reduction. H2 oxidises to H+,
leaving an acid medium that could interact with the basic N of the
quinuclidine ring of the anchored inducer if not neutralised. Table 1
summarises the values obtained for the surface concentration of the
modifier on the catalysts. We noted a drop in the TMS-CD content in
relation to the pure support attributable to the catalyst preparation
conditions, which involved leaching of the inducer and the corre-
3.2. Catalyst synthesis and characterisation
The catalysts were synthesized by H2 reduction of a soluble plat-
inum precursor using a pressure of 40 bar as reported by Roucoux
et al. [38]. The reaction at room temperature allowed us to obtain
Fig. 4. NMR of the supports modified with TMS-CD. (a) 13C and (b) 29Si (CP).

C.H. Campos et al. / Journal of Molecular Catalysis A: Chemical 348 (2011) 30–41
35
Table 2
Surface area, pore volume, average pore diameter and metal particle size of 1 wt%Pt/SiCD(x) catalysts.
a
a
Catalyst
SBET (m2 g−1
)
Pore volume (cm3 g−1
)
Average pore diameter (nm)
dXRD (nm)
dp TEM (nm)
Pt/SiCD(0.0)
Pt/SiCD(0.5)
Pt/SiCD(2.5)
Pt/SiCD(5.0)
Pt/SiCD(7.5)
Pt/SiCD(10)
337
306
300
296
288
282
0.88
0.82
0.75
0.75
0.71
0.69
10.6
10.4
10.2
10.1
9.8
10.0
7.5
6.8
6.0
5.7
5.3
9.6
8.0
4.3
3.4
3.0
2.6
9.7
a
Catalyst data.
of the type Pt␦+ became more intense, due mainly to the interac-
tion of the particles protected by TMS-CD with the N atoms of the
modifier [39,40]. A decrease was observed in the Pt/Si surface ratio
due to the surface coverage of SiO2 by TMS-CD at nominal contents
higher than 5.0 mmol CD/Lg, as shown in Table 3. Nonetheless, the
N/Si ratio increased with the CD content at the surface, as shown
by AE and DRIFT. The decline of the atomic ratio of Pt/N in the cata-
lysts suggests that as the TMS-CD content on the surface increased,
nanoparticles were deposited on sites around the anchoring sites,
causing a decrease in metallic particle size, as observed by XRD and
TEM.
sponding impurities. The C (%) and H (%) contents decreased, as did
those of N (%), since H2 is capable of reducing the (CH3)3–Si–O–Si
groups anchored to SiO2, releasing impurities and recovering the
silanol groups [48] and the basic medium is able to interact with
the anchored Si–O–Si bonds, releasing TMS-CD to a lesser degree
than observed for the impurities. Despite the clean surface of the
support during the catalyst preparation, the C/N ratio decreased,
coming quite near the nominal C/N values for SiCD(0.5) to SiCD(5.0).
The anchoring of impurities on SiCD(7.5) and SiCD(10) were further
from the nominal values, as shown in Fig. 1. In all cases, the H/N
ratio was comparable because the catalyst suffered a loss of impuri-
ties along with organic H. However, in all the cases, the H:N content
was always higher than the nominal values.
The DRIFT spectra and N2 adsorption–desorption isotherms of
the supports and catalysts did not exhibit significant differences, so
the crystalline nature of the active phase dispersed in the support
was studied by XRD (Fig. 6). Note the characteristic Pt diffraction
lines in the region of 2ꢁ ∼ 40◦. As the TMS-CD content at the surface
increased, the lines diminished in intensity, becoming less clear.
This was attributed to a higher dispersion of the active phase on
the surface of the support. The metal particle size, shown in Table 2,
was determined by XDR and TEM. All the catalysts exhibited metal-
lic particle sizes smaller than 10 nm. With HR-TEM, similar values
were obtained, as can be seen in the micrograph in Fig. 7. When the
catalyst was prepared on the activated support, particle sizes were
larger, reaching approximately 9.6 nm and a wider distribution of
particle sizes was observed. When TMS-CD was anchored to the
support, the average sizes declined as the organic molecule pro-
vided stability to the Pt cluster formed during the metal reduction,
supporting the formation of smaller-sized agglomerates. Given
these catalyst preparation conditions, we expected the nanopar-
ticles to show a preferential orientation towards the sites modified
with TMS-CD.
According the characterisation results strong interaction
between the active phase and TMS-CD should be expected because
of the significant changes in the metal particle size due to their
preferential deposition on the modified sites of the chiral inducer.
3.3. Catalytic activity
Fig. 9 shows catalytic activity curves. For all catalysts, the
activity varied with the TMS-CD content in the support. In all
the cases, we found pseudo-first-order kinetics with respect to
PPD, reaching conversion levels over 90% after 240 min for the
catalysts 1%Pt/SiCD(x) with x > 0.5 and below 60% for those of
x < 0.5. Scheme 2 shows the hydrogenation reaction of the car-
bonyl groups of the substrate. All the catalysts were selective for
(R)-1-hydroxy-1-phenylpropan-2-one (1R-FP) and (S)-1-hydroxy-
1-phenylpropan-2-one (1S-FP). Products of over hydrogenation
were not detected.
The catalyst Pt/SiCD(0.0) showed the least activity, mostly due
to the large size of the Pt crystals. A similar character was observed
in the case of 1%Pt/SiCD(0.5): using DRIFT, 29Si NMR, and SBET, the
surface characteristics of this catalyst were found to differ min-
imally from those of the activated support. However, it displays
higher conversion due to the smaller-sized Pt crystals because, as
discussed earlier, the anchoring of TMS-CD helped improve the
dispersion of the active phase on the support. In parallel, this cat-
alyst exhibits the deposition of impurity moieties which may be
considered poisons. These can interfere with the adsorption of the
substrate in the modified active phase (see Scheme 3), as reported
in analogous studies of the anchoring of silane derivatives on Pt
catalysts [49]. Although SBET decreased as the quantity of anchored
TMS-CD increased, there was a decline in the average metallic par-
ticle size. The resulting stabilisation effect allowed the deposition
of smaller-sized nanoclusters, which increased the conversion lev-
els (Table 4) and the rate constants, kg. The increased rates with
increased anchoring of TMS-CD on the supports can clearly be
observed. The catalysts supported on SiCD(x) with x > 0.5 exhib-
ited the highest conversion, reaching 97% in the case of SiCD(7.5).
This was justified by the lower-than-nominal C/N ratios, denoting
higher surface poisoning by impurities, in turn suggesting more
expeditious surfaces for active site–substrate interactions during
hydrogenation.
Fig. 8 shows the XPS core-level spectra obtained for the cata-
lysts. The peak corresponding to Pt0 at a BE of 71.0 eV was visible in
all the spectra. With greater TMS-CD contents, the peak of species
Fig. 6. XRD patterns of the catalysts synthesized on modified supports.

36
C.H. Campos et al. / Journal of Molecular Catalysis A: Chemical 348 (2011) 30–41
Fig. 7. TEM micrograph of the catalysts 1 wt%Pt supported on the modified SiO2 (a) SiCD(0.0); (b) SiCD(0.5); (c) SiCD(2.5); (d) SiCD(5.0); (e) SiCD(7.5) and (f) SiCD(10).

C.H. Campos et al. / Journal of Molecular Catalysis A: Chemical 348 (2011) 30–41
37
Table 3
Binding energies (eV) of internal electrons and atomic surface ratio of 1wt%Pt/SiCD(x) catalysts.
Catalyst
Pt 4f7/2 (eV)
N 1s (eV)
Si 2p (eV)
103.4
Pt/Si at
0.0025
N/Si at
–
Pt/N at
71.9 (94)
73.7 (6)
399.5 (64)
401.8 (36)
–
Pt/SiCD(0.0)
71.9 (93)
73.9 (7)
399.4 (67)
401.7 (33)
103.4
103.4
103.4
103.4
103.4
0.0133
0.0084
0.0104
0.0069
0.0070
0.0194
0.0312
0.0409
0.0490
0.0503
0.685
0.269
0.254
0.168
0.139
Pt/SiCD(0.5)
Pt/SiCD(2.5)
Pt/SiCD(5.0)
Pt/SiCD(7.5)
Pt/SiCD(10)
71.9 (82)
73.9 (18)
399.4 (71)
401.9 (29)
72.0 (77)
73.8 (23)
399.5 (62)
401.8 (38)
72.0 (83)
73.8 (17)
399.5 (65)
401.9 (35)
71.9 (77)
73.6 (23)
399.5 (63)
401.9 (37)
Scheme 2. Hydrogenation route of 1-phenylpropane-1,2-dione.
Scheme 3. Model of catalyst surface.

38
C.H. Campos et al. / Journal of Molecular Catalysis A: Chemical 348 (2011) 30–41
Table 4
Catalytic data for PPD hydrogenation. Reaction conditions: PPD concentration:
0.01 mol/L, PH : 40 bar, stirring speed: 700 rpm.
2
Catalyst
kg (102 min−1 g−1
)
Conversiona (%)
ee max (%)
Pt/SiCD(0.0)
Pt/SiCD(0.5)
Pt/SiCD(2.5)
Pt/SiCD(5.0)
Pt/SiCD(7.5)
Pt/SiCD(10)
3.7
8.2
41.7
48.9
68.3
67.6
56
67
95
95
97
96
–
37
45
42
47
40
a
At 4 h of reaction.
In relation to the selectivity of the catalysts, enantiomeric excess
and regioselectivity have been defined as:
[R] − [S]
Fig. 9. Activity curves based on PPD for all the synthesized catalysts. Reaction con-
ee (%) =
· 100
[R] + [S]
ditions: PPD concentration: 0.01 mol/L, catalyst mass: 0.050 g, PH : 40 bar, mixing
2
speed: 700 rpm, solvent: cyclohexane.
[RC1] + [SC1
[RC2] + [SC2
]
]
rs =
3.3.1. Recycling studies
The catalyst supported on SiCD(7.5) exhibited the highest ee
and was selected for assays of stability, activity, and selectivity in
consecutive batch cycles. Between cycles, the catalyst was washed
with chloroform in order to dissolve most of the organic matter that
could be adsorbed on the support. The studied catalysts were char-
acterised after each catalytic cycle by AE and XPS and the obtained
results are compiled in Table 5. In the case of TMS-CD leaching, the
organic matter was eliminated, as it is known to be highly solu-
ble in this solvent [2]. C (%) and H (%) were found to increase after
recycling. This was attributed to intermediate species of the sup-
port with 1R-FP and 1S-FP that appeared during the formation of
bonds of the type Si–O–R, as reported by Blümel [44] and/or to
the presence of species heavily adsorbed from the substrate, prod-
ucts, and/or solvent. N (%) decreases gradually along the catalytic
cycles, which is assigned to a partial leaching of the inducer. XPS
shows the presence of two types of surface nitrogen which may be
assigned to aliphatic (399.5 eV) and aromatic nitrogen (401.8 eV)
according Gammon et al. [52]. The inducer lost by leaching is not
re-adsorbed on the metallic sites because no changes in the binding
energy of the aromatic nitrogen was detected. On the other hand,
the remaining anchored chiral inducer is slightly hydrogenated
in the quinolline cycle of the TMS-CD as revealed the observed
enhancement in the aliphatic/aromatic ratio, as shown in Scheme 4.
The Pt/Ptı+ atomic surface ratio remains constant along the cycles
where [R] and [S] correspond to the concentrations of the respec-
tive enantiomers and in the case of rs, to the alcohols of the different
carbonyl groups. Fig. 10 shows the curves of enantiomeric excess
as a function of the conversion. All the catalysts were 100% selec-
tive for the hydrogenation of C O adjacent to the phenyl group
(see Scheme 2), giving 1R-FP as a product and an ee similar to the
results reported for systems in which the chiral auxiliary was added
in situ and adsorbed on the surface of the catalyst [50,51]. As the
TMS-CD content on the surface increased, the ee also increased,
reaching a maximum of 49% for the catalyst 1%Pt/SiCD(7.5), the
most active and enantioselective. This was due mainly to the polari-
sation caused by the inducer on the Pt nanoparticle. Upon increased
TMS-CD content, more Ptı+ species were observed by XPS (see
Fig. 8), favouring the selectivity of the product of interest.
Fig. 10. Curves of enantiomeric excess in function of the conversion of PPD for all the
synthesized catalysts. Reaction conditions: PPD concentration: 0.01 mol/L, catalyst
mass: 0.050 g, PH : 40 bar, mixing speed: 700 rpm, solvent: cyclohexane.
Fig. 8. XPS spectra for the catalysts supported on SiCD(x).
2

C.H. Campos et al. / Journal of Molecular Catalysis A: Chemical 348 (2011) 30–41
39
Table 5
Elemental analysis and XPS data of 1%Pt/SiCD(7.5) catalysts for cycles studies in the enantioselective hydrogenation of PPD.
Run
C (%)
4.01
3.97
4.83
5.63
7.04
9.06
H (%)
0.82
0.85
0.94
1.01
1.17
1.30
N (%)
0.51
0.45
0.42
0.41
0.41
0.40
Concentration
(10−4 mol/L g)
N 1s (eV)
Pt 4f7/2 (eV)
N/Si at
0.049
0.049
0.048
0.049
0.038
0.037
Pt/Si at
0
3.6
3.2
3.0
2.9
2.9
2.8
399.5 (65)
401.9 (35)
72.0 (83)
73.8 (17)
0.0069
0.0069
0.0064
0.0065
0.0052
0.0038
1
399.6 (64)
401.8 (36)
71.9 (79)
73.9 (21)
2
399.6 (68)
401.8 (32)
71.9 (79)
73.9 (21)
3
399.6 (70)
401.8 (30)
71.9 (75)
74.0 (25)
4
399.5 (71)
401.8 (29)
72.0 (78)
74.0 (22)
399.5 (74)
401.8 (26)
71.9 (75)
73.9 (25)
Final
Scheme 4. Hydrogenation of the anchored inducer.
whereas the Pt/Si ratio decreases, attributed to a slight leaching of
the metallic nanoparticles.
Fig. 11 shows the variations of conversion and ee as a func-
tion of the cycles studied. As the number of recycles increased, the
activity decreased, falling to values of 88%, as shown in Table 6.
Surface enrichment by undesired species on the support led, in
the second cycle, to lower selectivity by the catalyst, interfering
with the adsorption of the substrate on the modified active site.
Consequently, hydrogenation at these sites produced a racemic
mixture, decreasing the values of ee (see Table 6). In the third cycle,
we detected the appearance of RR-diols, SS-diols, and meso-diols,
as well as an ee that was attributed to the hydrogenation of the
Table 6
Catalytic data for cycles of the catalyst 1%Pt/SiCD(7.5). Reaction conditions: PPD concentration: 0.01 mol/L, catalyst mass: 0.050 g, PH : 40 bar, stirring speed: 700 rpm.
2
Run
Conversion max (%)
eea max (%)
eeb max (%)
rs max (%)
Concentration diolsa (103 mol/L)
0
1
2
3
4
97
93
92
90
88
49
37
40
43
54
–
–
–
–
–
–
–
2.0
2.1
2.2
3.1
47*,(30)
54*,(90)
45*,(15)
55*,(30)
a
ee referring to the C O of the C1 of PPD.
ee referring to the C O of the C2 of PPD.
b
*
,(x)Maximum value of ee and rs considering x: time in which only the products 1-hydroxy-1-phenyl-propane-2-one and 2-hydroxy-1-phenyl-propane-1-one were
observed.

40
C.H. Campos et al. / Journal of Molecular Catalysis A: Chemical 348 (2011) 30–41
variations as the TMS-CD content increased on the supports. All the
catalysts were active in the enantioselective hydrogenation of PPD
and selective for 1R-FP. The most enantioselective was that which
corresponded to a concentration of 0.75 mmol/L g. In consecutive
batch cycles, this catalyst lost activity and enantioselectivity due
to the interaction of the support with other species in the reaction
system between catalytic cycles.
Acknowledgements
The authors thank FONDECYT Project 1061001 and Project AT-
24091014.
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