Platinum supported on TiO2 as a new selective catalyst on heterogeneous hydrogenation of α,β-unsaturated oxosteroids

Article abstract of DOI:10.1016/j.molcata.2010.09.019

This paper describes the photochemical deposition of platinum onto TiO ₂ surface and the full characterization of the catalysts using BET, XPS and TEM techniques. The 1 and 3 wt. % Pt/TiO₂ catalysts reveal high activity with 70% and 96% of α-diastereoselectivity in the hydrogenation of the carbon-carbon double bond of the α,β-unsaturated oxosteroids 4-androstene-3,17-dione, and 3β-acetoxypregna-5,16-dien-20- one, respectively. Using higher temperature and pressure, the catalysts promote the further reduction of C-3 carbonyl group. Catalyst recovering and recycling is easily achieved with no appreciable lost of catalytic activity after five subsequent runs.

Full text of DOI:10.1016/j.molcata.2010.09.019

Journal of Molecular Catalysis A: Chemical 333 (2010) 1–5
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Editor’s choice paper
Platinum supported on TiO as a new selective catalyst on heterogeneous
2
hydrogenation of ␣,␤-unsaturated oxosteroids
a
b
a,∗
c
b,∗
Rui M.D. Nunes , Bruno F. Machado , Mariette M. Pereira , Maria José S.M. Moreno , Joaquim L. Faria
a
Departamento de Química, Universidade de Coimbra, Rua Larga, 3004-535 Coimbra, Portugal
b
Laboratório de Catálise e Materiais (LCM), Laboratório Associado LSRE/LCM, Departamento de Engenharia Química, Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto
Frias s/n, 4200-465 Porto, Portugal
c
Centro de Estudos Farmacêuticos, Faculdade de Farmácia, Universidade de Coimbra, 3000-295 Coimbra, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 June 2010
Received in revised form
This paper describes the photochemical deposition of platinum onto TiO2 surface and the full characteri-
zation of the catalysts using BET, XPS and TEM techniques. The 1 and 3 wt. % Pt/TiO2 catalysts reveal high
activity with 70% and 96% of ␣-diastereoselectivity in the hydrogenation of the carbon–carbon double
bond ofthe ␣,␤-unsaturatedoxosteroids4-androstene-3,17-dione, and3␤-acetoxypregna-5,16-dien-20-
one, respectively. Using higher temperature and pressure, the catalysts promote the further reduction
of C-3 carbonyl group. Catalyst recovering and recycling is easily achieved with no appreciable lost of
catalytic activity after five subsequent runs.
2
5 September 2010
Accepted 28 September 2010
Available online 7 October 2010
Keywords:
©
2010 Elsevier B.V. All rights reserved.
Heterogeneous
Hydrogenation
Unsaturated oxosteroid
Platinum
1
. Introduction
homogeneous catalysts onto solid supports [15]. Due to easily
catalyst recovery, heterogeneous catalysts have been extensively
employed in steroid hydrogenation, namely palladium supported
on carbon [16]. Also, heterogeneous catalysts serve the purpose of
diastereo- [17,18] and enantioselective [19] reduction of carbonyl
groups in the steroidal framework.
Chemo-selective
hydrogenation
of
multi-unsaturated
molecules over heterogeneous catalysts is a demanding task
1–4]. The polyfunctionalization of the steroidal framework, for
[
example, is a stimulating stereochemical exercise in synthetic
chemistry. The complexity and diversity of functional groups
endow these molecules with their diverse biological properties,
namely as hormones and vitamins [5]. The characterization of
the receptors and enzymes involved in the biosynthesis and
metabolism of these compounds engaged the synthetic chemist
on the optimization and development of stereoselective processes.
Therefore, the chemo and diastereoselective hydrogenation of
unsaturated steroids remains an important field due to the poten-
tiality of this process to synthesize steroidal bioactive molecules
and metabolites.
The diastereoselectivity of these hydrogenation reactions
depends on the mode of substrate adsorption 1,2 versus 1,4 to
the catalytic active site, which can be modulated by metal, sup-
port, solvent polarity and pH [16,20–24]. In general, heterogeneous
4
reduction of ꢀ -3-ketosteroids produces mainly the 5␤-isomer
[25–27], while the homogeneous or supported homogeneous cat-
alytic processes allow the opposite ␣-selectivity [5,8,28].
Titanium dioxide can be used as a catalyst itself, photocatalyst
under UV light in the degradation of organic pollutants [29–33], or
as a support for heterogeneous catalyst preparation. In this field,
noble metals, especially platinum, have been used to some extent.
The possibility of taking advantage of the strong metal support
interaction (SMSI) effect, after high temperature reduction, cer-
tainly makes TiO2 a very interesting support for hydrogenation
reactions [34].
In the literature, several approaches are reported to perform the
selective reduction of steroidal conjugated enones and dienones
by using metal–ammonia solutions [6,7] and catalytic processes,
either homogeneous [8,9], heterogeneous [10–14] or by linking
The photochemical deposition is an efficient method for metal
deposition over semiconductor materials [35,36], which was suc-
cessfully applied in the recovery of noble metals from wastewaters
under UV light [30,32], and can be used as an alternative tech-
∗ Corresponding authors. Tel.: +351 225 081 645; fax: +351 225 081 449.
E-mail addresses: mmpereira@qui.uc.pt (M.M. Pereira), jlfaria@fe.up.pt
J.L. Faria).
(
1
381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.09.019

2
R.M.D. Nunes et al. / Journal of Molecular Catalysis A: Chemical 333 (2010) 1–5
nique to the preparation of noble metal supported catalysts. Its
main advantage resides on the ability of spreading very effectively
the metal throughout the support, thus normally leading to high
mixed Lorentzian–Gaussain functions after a Shirley background
subtraction.
2.2. Catalyst synthesis
dispersions. Additionally, when a semiconductor, like TiO , is used
2
as support the photodeposition process also results in simultane-
ous reduction of the metal ions by conduction band electrons. The
latter can be further enhanced by addition of certain substances
said sacrificial electron donors that can supply an almost unlimited
amount of electrons and, thus, improve the rate of photodeposition.
Among the most commonly used are formaldehyde, methanol and
Titania supported platinum catalysts with different metal load-
ings, 1 and 3 wt. % (1Pt/TiO2 and 3Pt/TiO , respectively), were
2
prepared by the photochemical deposition method. The aqueous
solutions containing the desired amounts of methanol (Riedel-
de Haën, 99.8), dihydrogen hexachloroplatinate (IV) (Alfa Aesar,
9
9.9%) and titanium dioxide (P-25, Evonik previously Degussa)
2
-propanol [37].
were sonicated for 30 min to prevent particle agglomeration. The
suspensions were then irradiated by a low-pressure mercury lamp
with an emission line at 253.7 nm (ca. 3 W of radiant flux) for 4 h.
The catalysts were then recovered by filtration and dried in oven
In the present study we report the photochemical deposition
of platinum into TiO2 and the full characterization of the catalyst.
These heterogeneous catalyst presented high activity and selec-
tivity for the carbon–carbon double bond hydrogenation of the
,␤-unsaturated oxosteroids 4-androstene-3,17-dione, 1, and 3␤-
acetoxypregna-5,16-dien-20-one, 2.
◦
◦
at 90 C for 2 days [35]. A 500 C thermal treatment was applied
to the catalysts, consisting of calcination under nitrogen (N , 4 h,
␣
2
−
1
−1
1
00 mL min ), reduction with hydrogen (H , 2 h, 20 mL min )
2
and finally flushing again with N2 for 30 min in order to remove
physisorbed hydrogen, and stored in a desiccator until further use.
The platinum loaded to the support was controlled indirectly by
monitoring the 261 nm band of the rinsing water using an UV–vis
spectrometer (Jasco V560).
2
. Experimental
2.1. Equipments and reagents
1H and 13C NMR spectra were recorded in CDCl3 solutions on
Bruker Avance 300 operating at 300.13 for H and 75.47 MHz for
C. TMS was used as internal reference. GC and GC–MS exper-
iments were carried out on Agilent 6890 series equipped with
capillary Agilent HP5 columns with 30 m and 0.5 m respectively,
and identification of products was carried out against standards
A 10 wt. % Pd/C commercial (10Pd/C, extent of labeling: 10 wt.
% loading in a dry basis, matrix activated carbon, wet support,
Degussa type E101 NE/W, Sigma–Aldrich 330108) was used as
benchmark catalyst for the reduction of 4-androstene-3,17-dione.
1
13
2.3. Hydrogenation procedure
[
15,28].
Solvents were obtained from commercial sources (Aldrich) dis-
The substrate (0.07 mmol) and the catalyst were introduced in
the autoclave. The system was sealed and then purged with three
H /vacuum cycles, after which toluene (10 mL) was introduced
through an inlet cannula. The reaction mixture was heated to the
required temperature and then the system was filled with desired
H2 pressure. After reaching the desired temperature and pressure,
stirring was initiated. The conversions and selectivity were deter-
mined by gas chromatography.
tilled and dried before use, according to standard procedures [38].
2
Specific BET surface areas (SBET) were calculated based on the
◦
nitrogen adsorption–desorption isotherms determined at −196 C
with a Coulter Omnisorp 100CX apparatus. The samples were out-
◦
gassed for at least 4 h at 150 C, before allowing them to cool down
in vacuum to ambient temperature.
The metal dispersion was determined by H2 chemisorption at
room temperature in an U-shaped tubular quartz reactor after a
thermal treatment to remove contaminant species from the cata-
lyst surface. Pulses of H2 were injected through a calibrated loop
into the sample at regular time intervals until the area of the peaks
became constant. The amounts of H2 chemisorbed were calculated
from the areas of the resultant H2 peaks. The H2 was monitored
with a SPECTRAMASS Dataquad quadrupole mass spectrometer.
Transmission electron microscopy (TEM) observations were
made with a LEO 906 E from LEICA (120 kV voltage). The samples
were dispersed in ethanol and collected on a copper carbon-coated
TEM grid.
2
.3.1. Reduction of 4-androstene-3,17-dione and 3ˇ-acetoxy-
pregna-5,16-diene-20-one
-Androstene-3,17-dione (1) and 3␤-acetoxy-pregna-5,16-
4
diene-20-one (2) were submitted to heterogeneous catalytic
hydrogenation according to the general procedure described above.
When the catalytic reaction was stopped, the reactor was cooled
and depressurized. Evaporation of toluene afforded a crude that
was submitted to silica gel preparative column chromatography
using CH Cl :AcOEt (9:1) as eluent.
2
2
5
␣-H-androstan-3,17-dione 1.1 and 3␤-acetoxy-17␣-H-pregn-
-ene 2.1 were isolated and characterized. Data is in good
agreement with the previously reported one [15,28].
5
Surface analysis for topographical and analytical characteriza-
tion was carried out by scanning electron microscopy (SEM) with
a JEOL JSM-6301F (15 keV) electron microscope equipped with an
OXFORD INCA ENERGY 350 energy dispersive X-ray spectroscopy
2
.4. Catalyst recycling
(
EDS) system. The sample powders were mounted on a double-
After each run, the reactor was opened to air. Catalyst recovery
sided adhesive tape and observed at different magnifications under
two different detection modes, secondary and back-scattered elec-
trons.
was performed by filtration of the solid from the reaction mixture.
Then, the catalyst was reintroduced in the reactor and purged. A
fresh substrate solution, in toluene, was introduced via an inlet can-
nula. The catalytic reaction conditions described above were used
to perform the subsequent catalytic cycles.
X-ray photoelectron spectroscopy (XPS) was used to deter-
mine the surface atomic composition of platinum supported
catalysts in a VG Scientific ESCALAB 200A spectrometer using a
non-monochromatized AlK␣ radiation (1486.6 eV). The sample
powders were mounted directly on a double-sided adhesive tape.
3. Results and discussion
−
8
The pressure in the analysis chamber was inferior to 10 mbar
during data collection. Binding energy (BE) spectra were recorded
in the regions of O1s, Ti2p, and Pt4f. The spectra thus obtained were
analyzed by XPSpeak 4.1 software by deconvoluting the peaks with
3.1. Catalyst characterization
P-25 titanium dioxide from Evonik was used throughout this
study and consists of an anatase/rutile (A/R) crystalline ratio of

R.M.D. Nunes et al. / Journal of Molecular Catalysis A: Chemical 333 (2010) 1–5
3
Table 1
Specific BET surface area (SBET), platinum particle size (dPt), load (yPt) and amount of reduced platinum (Pt(0)) for TiO2 supported catalysts.
2
−1
dPta (± 0.2 nm)
dPtb (± 5 nm)
Pt(0) (± 3%)
Catalyst
TiO2
SBET (± 5 m
g
)
yPt (± 0.05 wt. %)
32.3
32.9
34.8
–
4.0
5.4
–
30
73
–
1.03
3.02
–
66
91
1
3
Pt/TiO2
Pt/TiO2
a
Determined by TEM analysis.
determined by H2 chemisorption.
b
2
−1
8
0/20, surface area of 32 m g and particle size ca. 22 nm. Due to
the production mechanism of this material TiO2 is essentially non-
porous with all the observed surface area due to adsorption on the
external surface. A certain load of Pt (1–3 wt. %) was deposited on
the surface by the photochemical method. Table 1 summarizes cat-
alyst specific BET surface areas, platinum particle size (according
to TEM analysis and hydrogen chemisorption), load (EDS semi-
quantitative analysis) and oxidation state (XPS analysis). For each
sample analyzed a minimum of 3 reproducible measurements was
considered.
Textural analysis of platinum catalysts did not reveal any sig-
nificant changes to the specific BET surface area, as the values are
very similar to that of the naked TiO2 support.
The Pt load agrees with the targeted amounts and in spite of
EDS being a surface technique, since the support is non-porous, the
determination offers a good representation of the real load. In addi-
tion, UV–vis analysis of the aqueous solution used in deposition,
confirmed that the full amount of metal was deposited.
The particle size determined by H2 chemisorption measure-
ments (Table 1) is considerably larger than that determined by
TEM analysis. These differences can be explained considering the
existence of a strong metal support interaction (SMSI) effect [34],
where the surface becomes decorated with sub-stoichiometric TiOx
moieties hindering H2 chemisorption to the noble metal. Hence,
the difference in particle size is attributed to a H2 chemisorp-
tion blockage instead of a sintering effect by the metallic phase
Fig. 1. TEM micrograph of 1Pt/TiO2 catalyst. Inset: alternative view of the same
(
Fig. 1).
According to XPS data no noticeable differences were observed
material at different magnification.
in the Ti 2p and O 1s characteristic regions between the naked and
Pt containing samples. The Ti 2p region was composed by the 2p3/2
and 2p1/2 doublet. The binding energy of Ti 2p3/2 peak was centered
3.2. Catalytic hydrogenation of ˛,ˇ-unsaturated oxosteroids
at 459.5, which can be assigned to Ti(IV), indicating that the Ti exists
In order to evaluate the activity and selectivity of the Pt/TiO2
catalysts, the hydrogenation of steroid 1 was performed with the
widely used and commercially available 10Pd/C catalyst (entry 6,
Table 2) and with the two different Pt/TiO2 catalysts, at different
values of temperature and pressure (Scheme 1a). The results are
collected in Table 2.
in the form of TiO . The peak centered at 530.4 eV was in good
2
agreement with O 1s electron binding energy for oxygen bound
to tetravalent Ti ions. For Pt, the XPS Pt 4f region can be resolved
into two pairs of doublets (Pt 4f7/2 at lower and Pt 4f5/2 at higher
binding energies). The XPS Pt 4f7/2 region shows a band that can
be deconvoluted into a peak centered at 71.3 eV assigned to Pt(0)
To perform the catalytic reaction, the suitable amount of
(
66% for 1Pt/TiO and 91% for 3Pt/TiO ), and other peak centered at
2
2
3Pt/TiO2 catalyst and substrate 1, dissolved in toluene, were
7
4.3 eV assigned to Pt(II), most likely in the form of PtO. All platinum
◦
introduced in the autoclave, at 100 C and 20 bar H . Gas chro-
2
peaks were resolved according to the constraint that the Pt 4f5/2 and
Pt 4f7/2 ratio was 6.81/8.65 [39].
matography analysis of aliquots, from the reaction mixture, showed
97% conversion with 95% of chemoselectivity for carbon–carbon
Table 2
−
3
Conversion and selectivity of the hydrogenation reactions [Pt] = 3.0 × 10 mmol; solvent: toluene (2 mL); substrate/M = 35 under several conditions of steroids 1 and 2.
◦
Entry
Steroid
Catalyst
P H2 (bar)
T ( C)
Time (h)
Conversion (%)
Chemo
Diastereo
selectivity (%)
selectivity (%)
1
2
3
1
1
1
3Pt/TiO2
3Pt/TiO2
3Pt/TiO2
20
20
5
100
30
30
1/12
1/2
1/2
97
94
96
45
98
95 (1.1 + 1.2)
95 (1.1 + 1.2)
97 (1.1 + 1.2)
97 (1.1 + 1.2)
96 (1.1 + 1.2)
85 (1.3)
97 (1.1 + 1.2)
96 (2.1)
79 (2.2)
71 (1.1)
70 (1.1)
70 (1.1)
73 (1.1)
75 (1.1)
1
2
8
/2
4
1
1Pt/TiO2
5
30
5
6
7
8
9
1
1
2
2
2
1Pt/TiO2
10Pd/C
1Pt/TiO2
1Pt/TiO2
TiO2
20
5
5
20
5
100
30
30
100
30
1/2
1
17
2
80
99
54 (1.1)
0
– (2.2)
Key for number designation in Scheme 1.

4
R.M.D. Nunes et al. / Journal of Molecular Catalysis A: Chemical 333 (2010) 1–5
Scheme 1.
double bond reduction and 71% of ␣-diastereoselectivity, after
min (entry 1, Table 2). To investigate the effect of temperature
of a chemical adsorption step in the mechanism. Thus, the some-
what unexpected high diastereoselectivity registered can result of
a differentiated adsorption of the conjugated functional groups in
the catalytic centre and its vicinity. In the case of the Pt catalyst
due to the SMSI effect (by which partial coverage of the metal
centre takes place), the alkene function is adsorbed in such a pref-
erential geometry that leads to the formation of the less hindered
diastereoisomer.
5
on the selectivity of the catalytic system the temperature was
◦
decreased to 30 C keeping the same pressure (entry 2, Table 2).
Even at this low temperature 3Pt/TiO2 showed high activity, 94%
after 30 min, but selectivity remained unchanged. The effect of
pressure on the activity and selectivity is negligible, as seen by
decreasing the hydrogen pressure from 20 bar (entry 2, Table 2)
to 5 bar (entry 3, Table 2) while the activity and selectivity are
unchanged.
The 1Pt/TiO2 catalyst was also used in the heterogeneous cat-
alytic hydrogenation of ꢀ5 -oxosteroid 2, at hydrogen pressure of
,16
◦
The effect of the amount of platinum adsorbed to TiO2 was also
5 bar and temperature of 30 C (Scheme 1b). After 1 h, 99% conver-
studied. The hydrogenation of steroid 1, with 1Pt/TiO , applying the
sion was achieved, with 100% regioselectivity for the ꢀ16 double
bond reduction and 97% of ␣-diastereoselectivity (entry 7, Table 2).
When the same substrate 2 was submitted to stronger hydro-
2
◦
milder reaction conditions (30 C and 5 bar), produced an almost
complete conversion, after 2 h, with the same selectivity as 3Pt/TiO2
◦
(
entries 4 and 3, Table 2, respectively).
genation conditions (20 bar and 100 C) and longer reaction time
16
5
It is also relevant to notice that when the hydrogenation of
(17 h), both ꢀ and the very hindered ꢀ double bonds were
reduced, as confirmed by 1H and C NMR (entry 8, Table 2). Even
at this more drastic reaction conditions, the 20 –CO group remains
unchanged.
13
steroid 1 is performed with substrate:catalyst = 300 (1Pt/TiO ;
2
◦
3
0 C and 5 bar) an almost complete conversion was obtained with
the same selectivity, after 26 h.
When the ␣,␤-unsaturated oxosteroid 1 was submitted to more
To evaluate the catalytic activity of the support TiO , a blank run
2
◦
◦
drastic reaction conditions (100 C, 20 bar H ) in the presence of
was carried out (5 bar and 30 C) using substrate 2. No reduction of
2
1
Pt/TiO , the carbonyl group at C-3 of the steroid ring was reduced
the less hindered ꢀ16 double bond was observed after 2 h, which
puts in evidence that the refractory material TiO2 does not present
any catalytic effect (entry 9, Table 2).
2
to the corresponding alcohols (85%), after 8 h (entry 5, Table 2).
Product characterization by 1H NMR, put in evidence that equato-
rial alcohols are predominantly formed (70%). Such outcome shows
that this heterogeneous catalytic system allows a similar selectivity
to the homogeneous rhodium-diphosphite catalyst but is signifi-
cantly more active [28].
3.3. Catalyst recycling
The recovery of the catalyst for reuse is of utmost environ-
mental and economical relevance, especially in diastereoselective
processes. In this context, using substrate 1, as model, we have
also assessed the effect of reusing the heterogeneous 3Pt/TiO2
catalyst on the activity and selectivity of the hydrogenation reac-
tion. Thus, after each catalytic run, the reactor was opened to the
air. The catalyst was then filtered and reintroduced in the reac-
tor, concomitantly with a new amount of substrate, keeping the
aforementioned catalyst/substrate ratio. The catalyst performance
remains unchanged after five subsequent runs (Fig. 2). It is note-
worthy that in all the catalytic cycles the chemoselectivity and
diastereoselectivity were not affected upon reutilization. Recycling
In order to compare the activity and selectivity of the new cata-
lyst (3Pt/TiO2 entry 3, Table 2) with the commercial 10Pd/C (entry
6
, Table 2), steroid 1 was submitted to the same hydrogenation con-
◦
ditions (5 bar; 30 C). The activity of both catalysts is similar but the
␣
-diastereoselectivity with 3Pt/TiO2 is significantly higher.
To estimate the activation energy, the reaction was carried out at
◦
different temperatures (from 30 to 50 C), at a constant pressure of 5
bar H and five or more aliquots were taken until complete conver-
2
sion was reached. From an Arrhenius plot of ln(k) as function of 1/T,
−1
the value of 78.4 kJ mol was obtained for the apparent activation
energy. This rather high value strongly supports the involvement

R.M.D. Nunes et al. / Journal of Molecular Catalysis A: Chemical 333 (2010) 1–5
5
This research was carried out under the projects
SFRH/BD/16966/2004, POCI/EQU/58252/2004, POCI/FEDER/2010
and, approved by the Funda c¸ ão para a Ciência e a Tecnologia (FCT)
and co-supported by FEDER (POFC/COMPETE).
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. Conclusions
[
[
The photochemical deposition method proved to be quite sim-
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4
16
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◦
◦
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The ␣-diastereoselectivity was outstanding, being comparable
to the homogeneous process. Such a result must be emphasized as,
according to the literature, surface unmodified heterogeneous cat-
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of the ␤-isomer.
[
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The authors would like to acknowledge Funda c¸ ão para a Ciên-
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(
RMDN) for financial support.