Hydrodeoxygenation of sulfoxides to sulfides by a Pt and MoOx co-loaded TiO2 catalyst

Article abstract of DOI:10.1039/c5gc02806j

Supported metal nanoparticle catalysts were studied for the hydrodeoxygenation of sulfoxides to sulfides under solvent-free and mild conditions (50-155 °C, 1 or 7 atm H₂). The catalytic activity for the model reaction of diphenyl sulfoxide depended on the type of metals, support materials and co-loaded oxides of transition metals (V, Nb, Mo, W, Re). Pt and MoO_x co-loaded TiO₂ (Pt-MoO_x/TiO₂) showed the highest activity. Pt-MoO_x/TiO₂ was reusable after the reaction and was effective for the reduction of various sulfoxides and showed a higher turnover number (TON) than previously reported catalysts. Using Pt-MoO_x/TiO₂, benzylphenylsulfone was reduced by H₂ to give phenylbenzyl sulfide via benzylphenyl sulfoxides, which represented the first example of catalytic conversion of a sulfone to a sulfide by H₂. Characterization studies of Pt-MoO_x/TiO₂ show that the surface of TiO₂ is covered by small (or thin layer) Mo oxide species with exposed Mo cations as Lewis acid sites, and 4-5 nm sized Pt metal nanoparticles are supported on the Mo oxide-covered TiO₂.

Full text of DOI:10.1039/c5gc02806j

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Hydrodeoxygenation of sulfoxides to sulfides by a
Pt and MoOx co-loaded TiO2 catalyst†
Cite this: DOI: 10.1039/c5gc02806j
Abeda Sultana Touchy,a S. M. A. Hakim Siddiki,b Wataru Onodera,a Kenichi Kona and
Ken-ichi Shimizu*a,b
Supported metal nanoparticle catalysts were studied for the hydrodeoxygenation of sulfoxides to sulfides
under solvent-free and mild conditions (50–155 °C, 1 or 7 atm H2). The catalytic activity for the model
reaction of diphenyl sulfoxide depended on the type of metals, support materials and co-loaded oxides
of transition metals (V, Nb, Mo, W, Re). Pt and MoOx co-loaded TiO2 (Pt–MoOx/TiO2) showed the highest
activity. Pt–MoOx/TiO2 was reusable after the reaction and was effective for the reduction of various sulf-
oxides and showed a higher turnover number (TON) than previously reported catalysts. Using Pt–MoOx/
TiO2, benzylphenylsulfone was reduced by H2 to give phenylbenzyl sulfide via benzylphenyl sulfoxides,
which represented the first example of catalytic conversion of a sulfone to a sulfide by H2. Characteri-
zation studies of Pt–MoOx/TiO2 show that the surface of TiO2 is covered by small (or thin layer) Mo oxide
species with exposed Mo cations as Lewis acid sites, and 4–5 nm sized Pt metal nanoparticles are sup-
ported on the Mo oxide-covered TiO2.
Received 23rd November 2015,
Accepted 4th January 2016
DOI: 10.1039/c5gc02806j
www.rsc.org/greenchem
deoxygenation of sulfones to sulfides, which is unprecedented
in the literature, is a challenging target in catalysis.
Introduction
Deoxygenation of sulfoxides to the corresponding sulfides is
Recently, we found that Pt and MoOx co-loaded TiO2 (Pt–
an important transformation in organic synthesis.1,2 Several MoOx/TiO2) showed high activity for three types of hydrogen-
catalytic methods have been developed to reduce sulfoxides ation reactions: (1) catalytic methylation of secondary amines
using stoichiometric amounts of reducing agents, including by CO2 and H2,25 (2) reductive amination of levulinic acid by
silanes,3–9 boranes,10,11 and phosphines,12–15 which suffer from H2 to N-alkyl-5-methyl-2-pyrrolidones,26 and (3) selective
the use of hazardous reagents and the production of by-pro- synthesis of primary amines by the reductive amination of
ducts that are sometimes difficult to remove from the reaction ketones.27 The results motivated us to study a possible appli-
media. Recently, greener catalytic methods with alcohols16–20 cation of Pt–MoOx/TiO2 to the reduction of SvO bonds.
as reducing agents were developed, but they also suffered from Considering that we have not characterized the structure of
low atom-efficiency. The catalytic deoxygenation of sulfoxides Pt–MoOx/TiO2 in detail, estimation of the structure of
by H2, as the most atom-efficient method, is a more challen- Pt–MoOx/TiO2 is an additional issue to be addressed.
ging reaction, but the previous catalytic methods21–23 suffer
We report herein a new catalytic system for the hydrodeoxy-
from limited substrate scope, low yields, low turnover number genation of various sulfoxides and sulfones to sulfides under
(TON) and no reports on catalyst reuse. Recently, Mitsudome mild conditions (50–155 °C, 1 or 7 atm H2) using Pt–MoOx/
et al.24 reported a heterogeneous catalytic system for the deoxy- TiO2 as a reusable heterogeneous catalyst. The substrate scope,
genation of various sulfoxides by 1 atm H2 using 5 mol% of catalyst reuse and catalyst characterization studies are also
Ru/TiO2, which exhibited a high TON (500) for hydrodeoxy- carried out to show the structure and catalytic performance of
genation of diphenyl sulfoxide. Further improvement in this Pt–MoOx/TiO2.
heterogeneous hydrodeoxygenation method will lead to a
greener route to sulfides from sulfoxides. Additionally, hydro-
Experimental
General
aInstitute for Catalysis, Hokkaido University, N-21, W-10, Sapporo 001-0021, Japan
Commercially available organic and inorganic compounds
bElements Strategy Initiative for Catalysts and Batteries, Kyoto University, Katsura,
(from Tokyo Chemical Industry, WAKO Pure Chemical Indus-
Kyoto 615-8520, Japan. E-mail: kshimizu@cat.hokudai.ac.jp; Fax: +81-11-706-9163
tries, Kanto Chemical or Mitsuwa Chemical) were used
without further purification. GC (Shimadzu GC-2014) and
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5gc02806j
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GCMS (Shimadzu GCMS-QP2010) analyses were carried out
In situ infrared (IR) spectra were recorded at 40 °C using a
with an Ultra ALLOY capillary column UA+-1 (Frontier Labora- JASCO FT/IR-4200 equipped with a quartz IR cell connected to
tories Ltd) using N2 and He as the carrier gas.
a conventional flow reaction system. The sample was pressed
into a 40 mg of a self-supporting wafer (ϕ = 2 cm) and
mounted into the quartz IR cell with CaF2 windows. Spectra
Catalyst preparation
TiO2 (JRC-TIO-4, 50 m2 g−1), MgO (JRC-MGO-3), CeO2 were recorded accumulating 30 scans at a resolution of 4 cm−1
.
(JRC-CEO-3) and HBEA zeolite (JRC-Z-HB25, SiO2/Al2O3 = 25
A reference spectrum of the catalyst wafer taken under He at
5) were supplied from the Catalysis Society of Japan. SiO2 measurement temperature was subtracted from each spectrum.
(Q-10, 300 m2 g−1) was supplied from Fuji Silysia Chemical For the IR study of pyridine adsorption on support materials
Ltd. Active carbon (C) was purchased from Kanto Chemical. (Fig. 1), the sample disc, pre-heated in He flow at 500 °C for
γ-Al2O3 was prepared by calcination of γ-AlOOH (Catapal B 0.5 h, was exposed to pyridine (1 μL as liquid) vaporized at
Alumina, Sasol) for 3 h at 900 °C. Nb2O5 was prepared by calci- 200 °C under He flow at 200 °C. After purging with He for 600 s,
nation of Nb2O5·nH2O (supplied by CBMM) at 500 °C for 3 h. the IR spectra of the adsorbed pyridine were obtained. For the
ZrO2 was prepared by hydrolysis of zirconium oxynitrate IR study of CO adsorption (Fig. 4), the disk of Pt-loaded catalysts
2-hydrate by an aqueous NH4OH solution, followed by fil- in situ pre-reduced under H2 (20 cm3 min−1, 300 °C, 0.5 h), was
tration, washing with distilled water, drying at 100 °C for 12 h, cooled to 40 °C under He, followed by flowing CO(5%)/He
and by calcination at 500 °C for 3 h.
(20 cm3 min−1) for 180 s. After purging with He (40 cm3 min−1
)
Precursors of M1-MoOx/TiO2 (M1 = 5 wt% Pt, Rh, Pd, Re, for 600 s, the IR spectrum of the adsorbed CO was obtained.
Ru, Ni, Cu; 7 wt% Mo) and Pt–M2Ox/TiO2 (5 wt% Pt; M2 = 7 wt
% Mo, V, Nb, W, Re) were prepared by a sequential impreg-
Catalytic tests
nation method using M1 source [aqueous HNO3 solutions of
Pt–MoOx/TiO2 was used as the standard catalyst. After the pre-
reduction at 300 °C, the catalyst in a closed glass tube with a
septum inlet was cooled to room temperature under H2.
n-Dodecane (0.05 g) was injected to the pre-reduced catalyst
inside the glass tube through the septum inlet, then the
septum was removed under air, and sulfoxides (1.0 mmol) and
a stirrer bar were charged to the tube, followed by inserting
the tube inside a stainless autoclave with a dead space of
28 cm3. Soon after being sealed, the reactor was flushed with
H2 and charged with 7 atm H2 at room temperature. Then the
reactor was heated at 50 or 120 °C under stirring (180 rpm) for
24 h. The reactions under ambient H2 pressure, eqn (1) and
(2), were carried out in a closed glass tube with balloon hydro-
gen at 155 °C. For the reactions in Table 1 and Fig. 1, 6–8 con-
versions and yields of sulfides were determined by GC using
n-dodecane as an internal standard adopting the GC-sensitivity
estimated using the commercial compounds. For the scope
and limitation study in Table 2, the isolated yields of products
were determined as follows. After the reaction, 2-propanol
Pt(NH3)2(NO3)2, Rh(NO3)3 or Pd(NH3)2(NO3)2, NH4ReO4, RuCl3
or aqueous solution of nitrates (Ni, Cu)], M2 source
[(NH4)6Mo7O24·4H2O,
NH4VO3,
niobium
oxalate,
(NH4)10W12O41·5H2O or NH4ReO4] and TiO2. For the prepa-
ration of Pt–MoOx/TiO2 (5 wt% Pt, 7 wt% Mo) as an example,
5 g of TiO2 and 0.88 mmol of (NH4)6Mo7O24·4H2O were added
to 50 mL of water at 50 °C, followed by evaporation to dryness
at 50 °C, and by drying at 90 °C for 12 h and calcination in air
at 500 °C for 3 h to obtain MoO3-loaded TiO2 (MoO3/TiO2).
MoO3/TiO2 was added to an aqueous HNO3 solution of Pt
(NH3)2(NO3)2, followed by evaporation to dryness at 50 °C, and
by drying at 90 °C for 12 h. Precursors of metal oxide-sup-
ported Pt catalysts were prepared by the impregnation method
using an aqueous HNO3 solution of Pt(NH3)2(NO3)2. Before
each catalytic experiment, catalysts were prepared by pre-
reduction of the precursor in a Pyrex tube under a flow of H2
(20 cm3 min−1) at 300 °C for 0.5 h.
Catalyst characterization
X-ray absorption near-edge structures (XANES) at the Pt L3-
edge were measured at the BL14B2 in the SPring-8 (Proposal
No. 2012A1734) in a transmittance mode. The storage ring was
operated at 8 GeV. A Si(111) double crystal monochromator
was used to obtain a monochromatic X-ray beam. Pt–MoOx/
TiO2 pre-reduced in 100% H2 (20 cm3 min−1) for 0.5 h at
300 °C was cooled to room temperature in H2 and was sealed
in cells made of polyethylene under N2, and then the XANES
spectrum was recorded at room temperature. XANES analysis
was performed using the REX version 2.5 program (RIGAKU).
The oxidation state of Mo species in the pre-reduced
Pt–MoOx/TiO2 was estimated by X-ray photoelectron spectroscopy
(XPS) using a JEOL JPS-9010MC (Mg Kα irradiation). Binding
energies were calibrated with respect to C1s at 285.0 eV.
TEM measurement of the pre-reduced Pt–MoOx/TiO2 was
carried out by using a JEOL JEM-2100F TEM operated at 200 kV.
Fig. 1 IR spectra of pyridine adsorbed on support materials (40 mg) at
200 °C.
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(4 mL) was added to the mixture, and the catalyst was sepa-
rated by centrifugation. Then, the reaction mixture was con-
centrated in a vacuum evaporator to remove the volatile
compounds. Then, sulfides were isolated by column chromato-
graphy using silica gel 60 (spherical, 63–210 μm, Kanto Chemi-
cal Co. Ltd) with hexane/ethylacetate (2/1 to 5/1) as the eluting
solvent, followed by analyses by 1H NMR, 13C NMR and GCMS.
The concentration of Pt in the solution after the standard reac-
tion was checked by inductive coupling plasma (ICP-AES)
using ICPE-9000 (Shimadzu).
Results and discussion
Characterization of Pt–MoOx/TiO2
Fig. 2 XPS spectra of the Mo 3d region of MoO3/TiO2 and Pt–MoOx/
TiO2.
First, we characterized the structure of the MoO3/TiO2 support.
The N2 adsorption result showed that the surface area of
MoO3/TiO2 was 54 m2 g−1. The XRD pattern of MoO3/TiO2
showed no diffraction lines due to Mo oxides (result not
shown), which indicate the absence of crystalline MoO3 par-
ticles on MoO3/TiO2. According to the literature,28 the non-
crystalline Mo(IV) oxide species on TiO2 are monolayer or small
clusters (polymeric molybdates) of Mo(IV) oxide. TEM images
of MoO3/TiO2 were also essentially identical to that of TiO2.
Fig. 1 compares the IR spectra (the ring-stretching region)
of pyridine adsorbed on MoO3/TiO2 and TiO2. For both of the
support materials, strong bands due to the coordinatively
bound pyridine on a Lewis acid (1445 and ca 1607 cm−1 29,30
)
are observed. The bands for MoO3/TiO2 are higher in intensity
than those for TiO2, which indicates that the relative amount
of Lewis acid sites of TiO2 is increased by the loading of MoO3.
It is known that the position of the band around 1607 cm−1
increases with the increase in the Lewis acid strength of metal
oxides.29 The higher wavenumber of the band for MoO3/TiO2
(1607 cm−1) than TiO2 (1604 cm−1) indicates that the Lewis
acid strength of TiO2 is increased by the loading of MoO3. Con-
sidering that the surface density of Mo on MoO3/TiO2 (7.4 Mo
atoms nm−2-TiO2) is larger than the monolayer coverage of
Mo on the TiO2 support (6 Mo atoms nm−2-TiO2),28 these
results suggest that the surface of TiO2 is covered by small (or
thin layer) polymeric molybdates whose surface contains coor-
dinatively unsaturated Mo cations as Lewis acid sites. This
structural model of MoO3/TiO2 is consistent with that in the
literature for MoO3/TiO2 with a monolayer coverage of Mo.28
Additionally, the spectrum for MoO3/TiO2 shows a broad and
weak band at1538 cm−1 due to the pyridinium ion (PyH+) pro-
duced by the reaction of pyridine with the Brønsted acid site.
Next, the structure of the representative catalyst, Pt–MoOx/
TiO2 pre-reduced at 300 °C, was characterized by various
spectroscopic methods. The oxidation state of Mo species was
studied by XPS. Fig. 2 compares the XPS spectra (Mo 3d5/2
region) of MoO3/TiO2 and Pt–MoOx/TiO2. As expected, the Mo
3d5/2 binding energy (BE) of MoO3/TiO2 (232.6 eV) corresponds
to the oxidation state of Mo6+.31,32 The Mo 3d5/2 peak of Pt–
MoOx/TiO2 appeared at a lower BE of 230.5 eV assignable to
Mo4+ species,31,32 but peaks due to the metallic Mo0 (227.9
Fig. 3 Pt L3-edge XANES spectra of Pt–MoOx/TiO2 and Pt foil.
eV)31 and PtMo alloys (227.8 eV)32 were not observed. Com-
bined with the structural model of MoO3/TiO2 discussed
above, the Mo species on the Pt–MoOx/TiO2 catalyst can be
small (or thin layer) MoO2 species.
The bulk oxidation state of Pt species in Pt–MoOx/TiO2 was
studied by Pt L3-edge XANES (Fig. 3). The XANES feature of Pt–
MoOx/TiO2 is quite close to that of the Pt foil, indicating the
metallic state of the Pt species in Pt–MoOx/TiO2. The surface
oxidation state of Pt species in Pt–MoOx/TiO2 was studied by
the IR spectra of CO adsorbed on the sample (Fig. 4). The
IR spectrum of CO adsorbed on Pt–MoOx/TiO2 showed a
strong band at 2073 cm−1 due to linearly coordinated CO on a
metallic Pt0 site32,33 together with a weak band due to bridged
CO adspecies on a Pt metal plane (1850 cm−1).32,33 This
indicates that the surface of Pt is metallic.
Fig. 5 shows the representative TEM images of Pt–MoOx/
TiO2. From low resolution (upper side) and high resolution
(lower side) TEM images, the Pt particle size distribution was
obtained as shown in the figure. The volume–area mean dia-
meter of Pt particles was 4.7 1.1 nm. This value is close to
the average diameter of Pt particles (4.1 nm)25 estimated by the
CO adsorption experiment assuming that CO is adsorbed on
the surface of spherical Pt particles at a stoichiometry of CO/
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Table 1 Catalyst screening for hydrogenation of diphenyl sulfoxide to
diphenyl sulfide
Entry
Catalysts
Conv. (%)
GC yield (%)
1
2
3
4
5
6
7
8
MoO3/TiO2
Pt–MoOx/TiO2
Pt–ReOx/TiO2
Pt–WOx/TiO2
Pt–NbOx/TiO2
Pt–VOx/TiO2
Pt–MoOx/Al2O3
Pt/Al2O3
Pt/Nb2O5
Pt/HBEA
Pt/MgO
Pt/CeO2
Pt/ZrO2
Pt/TiO2
Pt/SiO2
Pt/C
Rh–MoO3/TiO2
Pd–MoO3/TiO2
Re–MoO3/TiO2
Ru–MoO3/TiO2
Ni–MoO3/TiO2
Cu–MoO3/TiO2
9
100
69
56
41
37
73
58
56
48
42
41
34
30
25
25
75
57
40
34
26
15
8
97
67
54
40
36
71
56
54
47
39
38
31
28
23
24
72
55
39
33
24
14
Fig. 4 IR spectra of CO adsorbed on Pt–MoOx/TiO2 at 40 °C.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
Optimization of catalysts
Table 1 shows the influence of catalyst composition on the
catalytic activity for hydrogenation of diphenyl sulfoxide to
diphenyl sulfide under solvent-free conditions under 7 atm H2
at 50 °C for 24 h using 0.1 mol% of the catalyst. Among
various Pt catalysts, including bimetal loaded TiO2 (Pt–MOx/
TiO2; M = 7 wt% Mo, V, Nb, W, Re; entries 2–6), Pt–MoOx co-
loaded Al2O3 (entry 7) and Pt-loaded metal oxides (entries
8–16), Pt–MoOx/TiO2 (entry 2) showed the highest yield of
diphenyl sulfide (97%) as well as the highest conversion of
diphenyl sulfoxide (100%). The yield for Pt–MoOx/TiO2 (97%) is
much higher than those for MoO3/TiO2 (8%) and Pt/TiO2 (28%),
which indicates a synergistic effect between Pt and MoO3. Pt–
MoOx/TiO2 showed a higher yield than the various metal (Rh,
Pd, Re, Ru, Ni, Cu)-loaded MoOx/TiO2 catalysts (entries 17–22).
Summarizing the screening result, it is found that Pt–MoOx/
TiO2 is the best catalyst in the 22 types of the heterogeneous cat-
alysts tested as shown in Table 1. Under the same conditions,
Pt–MoOx/TiO2 catalysts with Pt loadings of 0.1 and 1 wt%
showed 61% and 80% yields (results not shown), which were
lower than the standard catalyst with 5 wt% Pt loading.
Fig. 5 Representative TEM images and Pt particle size distribution of
Pt–MoOx/TiO2. The volume–area mean diameter of Pt particles was 4.7
1.1 nm.
In our previous studies on the hydrodeoxygenation of carbo-
nyl compounds by Pt-loaded catalysts, we showed that the cata-
lytic activity was highly support-dependent; Lewis acidic
supports (such as MoOx/TiO2) showed a higher activity than the
other supports.26,27 The role of the Lewis acid sites is shown to
be the activation of CvO bonds by Lewis acid–base interactions
between the surface Mo cation and carbonyl oxygen, while the
(surface Pt atom) = 1/1. Summarizing the structural results, the
dominant Pt species in Pt–MoOx/TiO2 are around 4–5 nm sized
Pt metal nanoparticles. Mo oxides are not observed in the high
resolution TEM image, which supports that MoO2 species on
Pt–MoOx/TiO2 are small clusters or thin layer species.
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Table 2 Pt–MoOx/TiO2-catalyzed sulfide synthesis from various
sulfoxides
Isolated
yield
(%)
Entry Sulfoxide
1a
Sulfide
97
Fig. 6 Catalyst reuse for hydrogenation of diphenyl sulfoxide to di-
phenyl sulfide.
2
3
92
88
4 cycles. ICP-AES analysis of the solution after the first reaction
showed that the Pt content in the solution was below the
detection limits. The results indicate that Pt–MoOx/TiO2 is a
reusable heterogeneous catalyst for this reaction.
As summarized in Table 2, the present solvent-free hydro-
genation method was applicable to various sulfoxides under 7
atm H2 at 50 or 120 °C. The reactions of aromatic (entries 1–7),
benzylic (entry 8) and aliphatic (entries 9 and 10) sulfoxides
resulted in the formation of the corresponding sulfides with
high isolated yields (85–97%). The method showed chemose-
lective hydrogenation of sulfoxides without the conversion of
other reducible functional groups: Cl– (entry 3), Br– (entry 7),
carbonyl (entry 5) groups.
It is important to note that the catalytic system shows a
high TON under ambient pressure (1 atm) of H2 in a glass
reactor under solvent-free conditions at 155 °C. As shown in
eqn (1), 1 mmol of diphenyl sulfoxide was hydrogenated by Pt–
MoOx/TiO2 containing 0.001 mmol (0.1 mol%) of Pt, and
diphenyl sulfide was obtained in 99% yield after 24 h. As
shown in eqn (2), hydrogenation of 5 mmol of diphenyl sulfox-
ide under 1 atm H2 for 120 h by a smaller amount of the cata-
lyst (0.02 mol%) gave diphenyl sulfide in 88% yield,
corresponding to a TON of 4400 with respect to the total
number of Pt atoms in the catalyst. This value is higher than
that of previously reported catalysts.21–24 However, the reaction
of 30 mmol of diphenyl sulfoxide with 0.01 mol% of the cata-
lyst for 120 h resulted in only 28% yield (not shown).
4
5
87
85
6
7
94
91
8b
96
9
98c
91
10
a T = 50 °C. b t = 36 h. c GC yield.
Pt is proposed to act as a H2 dissociation site.26,27 We speculate
the same cooperative mechanism of Pt and Lewis acid sites in
the present catalytic system, in which Mo cations as Lewis acid
sites activate SvO bonds in the substrates.
Catalytic performance of Pt–MoOx/TiO2
ð1Þ
As shown in Fig. 6, the Pt–MoOx/TiO2 catalyst showed good
reusability for the hydrogenation of diphenyl sulfoxide under
the conditions given in Table 1. After the reaction, 2-propanol
(4 mL) was added to the mixture, and the catalyst was separ-
ated by centrifugation. The catalyst was dried at 100 °C for 1 h
and reduced under H2 at 300 °C for 0.5 h. The recovered cata-
lyst showed high yields (>89%) of diphenyl sulfide for the next
ð2Þ
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a profile characteristic of a consecutive reaction pathway. The
yield of unreacted benzyl phenyl sulfone decreased with time.
The yield of the partially hydrogenated product (benzyl phenyl
sulfoxides) increased with time and then decreased. After 40 h,
a completely deoxygenated product (benzyl phenyl sulfide) was
selectively obtained in 85% yield. To our knowledge, the result
represents the first example of catalytic hydrogenation of a
sulfone to a sulfide by H2.
Conclusions
Fig. 7 Initial formation rate of diphenyl sulfide vs. concentration of a
sulfide, 4-(methylthio)aniline, for hydrogenation of diphenyl sulfoxide to
diphenyl sulfide at 50 °C under 7 atm H2 by 0.1 mol% Pt–MoOx/TiO2.
Characterization of Pt–MoOx/TiO2 showed that the surface of
TiO2 was covered by small (or thin layer) MoO2 species with
exposed Mo cations as Lewis acid sites and 4–5 nm sized Pt
metal nanoparticles were loaded on the support. Pt–MoOx/
TiO2 was found to be an effective and reusable catalyst for the
reduction of sulfoxides to sulfides under solvent-free and mild
conditions (50–155 °C, 1 or 7 atm H2), which showed a higher
TON than previously reported catalysts. Pt–MoOx/TiO2 cata-
lyzed the reduction of benzyl phenyl sulfone by H2 to benzyl
phenyl sulfide, which represented the first example of catalytic
conversion of a sulfone to a sulfide by H2.
Acknowledgements
This work was supported by Grant-in-Aids for Scientific
Research B (26289299) from MEXT (Japan), a MEXT program
“Elements Strategy Initiative to Form Core Research Center”
and a Grant-in-Aid for Scientific Research on Innovative Areas
“Nano Informatics” (25106010) from JSPS.
Fig. 8 Time dependence of GC yields for the reduction of benzyl
phenyl sulfone at 120 °C under 7 atm H2 by 0.1 mol% Pt–MoOx/TiO2
●
catalyst: ( ) unreacted benzyl phenyl sulfone, (∇) benzyl phenyl sulfo-
References
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In general, transition metal surfaces interact strongly with
sulfur atoms in sulfides, resulting in the deactivation of the
catalysts. To test the sulfur-tolerance of the Pt–MoOx/TiO2 cata-
lyst during the catalytic reaction, we studied the effect of the
concentration of a model sulfide, 4-(methylthio)aniline, on the
initial formation rate of diphenyl sulfide for the hydrogenation
of diphenyl sulfoxide by Pt–MoOx/TiO2 (Fig. 7). The reaction
rate slightly decreased with the sulfide concentration, but the
reaction order with respect to the sulfide (n = −0.09) was close
to zero. The result indicates that the Pt–MoOx/TiO2 catalyst
shows a moderate sulfur-tolerance during the reaction. This
sulfur-tolerance may be a possible reason for the high TON of
this catalytic system.
Interestingly, the present catalytic system was found to be
effective for the hydrogenation of a sulfone. Fig. 8 shows a
time–yield profile for the reduction of benzyl phenyl sulfone
under 7 atm H2 in the presence of 0.1 mol% of the Pt–MoOx/
TiO2 catalyst (containing 0.001 mmol of Pt). The result shows
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