Efficient hydrodeoxygenation of sulfoxides into sulfides under mild conditions using heterogeneous cobalt-molybdenum catalysts

Article abstract of DOI:10.1039/c9gc02465d

Nitrogen-doped carbon-supported cobalt-molybdenum bimetallic catalysts (abbreviated as Co-Mo/NC) are active for the hydrodeoxygenation of sulfoxides to sulfides under mild conditions (25-80 °C and 10 bar H₂), which represents the first example of the use of heterogeneous non-noble metal catalysts for this transformation. MoO₃ with Lewis acid sites assists the hydrodeoxygenation of sulfoxides into sulfides by hydrogen over cobalt nanoparticles.

Full text of DOI:10.1039/c9gc02465d

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Efficient hydrodeoxygenation of sulfoxides
into sulfides under mild conditions using
Cite this: Green Chem., 2020, 22, 39
heterogeneous cobalt–molybdenum catalysts†
Received 18th July 2019,
Accepted 12th November 2019
Kaiyue Yao,‡ Ziliang Yuan,‡ Shiwei Jin,* Quan Chi, Bing Liu, * Renjie Huang and
Zehui Zhang
DOI: 10.1039/c9gc02465d
rsc.li/greenchem
Nitrogen-doped carbon-supported cobalt–molybdenum bi- lysts were earlier utilized for the hydrodeoxygenation of dimethyl
1
0,11
metallic catalysts (abbreviated as Co–Mo/NC) are active for the sulfoxide,
hydrodeoxygenation of sulfoxides to sulfides under mild con- and there is the purification of the products to be considered.
ditions (25–80 °C and 10 bar H ), which represents the first Obviously, the use of heterogeneous catalysts can address the
example of the use of heterogeneous non-noble metal catalysts drawbacks associated with homogeneous catalytic systems.
for this transformation. MoO with Lewis acid sites assists the However, the deoxygenation of sulfoxides with H is unpre-
hydrodeoxygenation of sulfoxides into sulfides by hydrogen over cedented over heterogeneous catalysts. In 2014, Mitsudome and
but it is difficult to recycle homogeneous catalysts
2
3
2
12
cobalt nanoparticles.
co-workers made a great contribution with the hydrodeoxygena-
tion of sulfoxide over a Ru/TiO catalyst, which was reported to be
active under atmospheric H2 pressure. Later, heterogeneous
2
The selective hydrodeoxygenation of sulfoxides into sulfides is
one of the most important organic transformations in organic
13
platinum catalysts (Pt–MoO /TiO and Pt/V0.7Cr0.3O ) were devel-
X
2
X
1
chemistry, which has attracted great interest in the fine
oped for the hydrodeoxygenation of sulfoxides, which was per-
chemical industry as well as in biochemistry. For example, sulf-
oxides are now widely utilized as chirons in numerous asym-
metric transformations wherein the sulfoxide groups, after
stereoselective introduction, are removed by deoxygenation to
their corresponding sulfides followed by hydrodesulfuriza-
14,15
formed at 50–150 °C under hydrogen pressure (1–7 bar).
Although heterogeneous noble metal catalysts are effective for
this transformation, the use of heterogeneous non-noble metal
catalysts is highly attractive because of the lower cost.
To the best of our knowledge, no heterogeneous non-noble
metal catalysts have been reported for this transformation. In
recent years, nitrogen-doped carbon material-supported non-
noble metal catalysts have received a great deal of attention in
the replacement of noble metal catalysts. Our group recently
discovered that nitrogen-doped carbon material-supported
2
tion. In the early days, the hydrodeoxygenation of sulfoxides
was performed by the use of excess amounts of sacrificial
agents such as metal hydrides and zinc metal,
3
–5
which
produce large quantities of waste. There have also been some
reports on metal-free deoxygenation of sulfoxides with halides,
such as iodides in the presence of acids. However, these
methods produce halide by-products, which are not environ-
mentally friendly.
Currently, catalytic hydrodeoxygenation represents an envir-
onmentally friendly method. Compared with other reducing
agents such as alcohols and silanes, hydrogen is a clean and
environmentally friendly reducing agent because of high atom-
efficiency with only the release of water. Homogeneous cata-
6
,7
cobalt catalysts demonstrate excellent activity in hydrogenation
16,17
and oxidation reactions.
Therefore, we also tried to use
nitrogen-doped carbon-supported cobalt catalysts for the
hydrodeoxygenation of sulfoxides, but failed. Inspired by the
fact that some metals such as Re, Mo and W are oxyphilic
elements, nitrogen-doped carbon material-supported Co–Mo
bimetallic catalysts were constructed, and found to be active
for the selective hydrodeoxygenation of sulfoxides.
8
9
ZIF-67 as one of the most investigated metal–organic frame-
works was used for the preparation of Co–Mo/NC-T
catalysts, where T represents the reduction temperature. In
order to introduce the oxyphilic element molybdenum,
Key Laboratory of Catalysis and Materials Sciences of Hubei, South-Central
University for Nationalities, Wuhan, 430074, China.
E-mail: liubing@mail.scuec.edu.cn, jinsw@mail.scuec.edu.cn; Fax: +86-27-67842572;
Tel: +86-27-67842572
(
4 6 7 2
NH ) Mo O24·4H O was introduced into ZIF-67 via a double-
solvent method, and the mixture was subjected to pyrolysis at
different temperatures (see the Experimental details in the
ESI†). Powder X-ray diffraction (PXRD) was performed to deter-
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9gc02465d
These authors contributed equally to this work.
‡
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Fig. 2 TEM image (a) and HR-TEM image (b) of the Co–Mo/NC-400
catalyst.
Fig. 1 PXRD patterns of the Co–Mo/NC-T catalysts.
mine the crystal structure of the Co–Mo/NC-T catalysts. As for the Mo species were found in the HR-TEM image of the
shown in Fig. 1, three characteristic peaks at 2θ = 44.2°, 51.5° Co–Mo/NC-400 catalyst.
and 75.9° corresponding to metallic Co (111), (200), (220)
X-ray photoelectron spectroscopy (XPS) was further used to
reflections were observed for the Co–Mo/NC-T catalysts (PDF study the valence states of the Co and Mo species in the Co–
1
8
1
5-0806). The average size of Co nanoparticles was estimated Mo/NC-T catalysts. Two peaks with binding energies at 781.4
to be 9.8, 28.5 and 33.9 nm from the line broadening of the and 796.6 eV were assigned to the Co 2p and Co 2p1/2 of
3
/2
2
+
Co (111) reflection for the Co–Mo/NC-400, Co–Mo/NC-600 and Co , while two peaks with binding energies at 779.5 eV and
Co–Mo/NC-800 catalysts, respectively. There were no XRD 795.0 eV were attributed to the Co 2p3/2 and Co 2p1/2 of Co .
peaks of molybdenum species for the Co–Mo/NC-400 and Two peaks with binding energies at 785.7 and 802.4 eV corre-
Co–Mo/NC-600 catalysts, while characteristic peaks at 2θ = sponded to the Co LMM auger peak in Co
4.5°, 38.1° and 39.5° were observed in the PXRD pattern of ence of Co in the Co–Mo/NC-400 catalyst was due to the oxi-
the Co–Mo/NC-800 catalyst, which were assigned to the (021), dation of surface metallic cobalt nanoparticles during storage
3
+
2
+
22
3
O
4
.
The pres-
3
3 4
O
1
9
(200), and (121) reflections of Mo
2
C. According to the in the air. Similarly, the surface of cobalt nanoparticles was
reported results, (NH ) Mo O24·4H O was gradually changed also present in the oxidation states for Co–Mo/NC-600 and Co–
1
9
4 6 7 2
into MoO from 300 °C to 500 °C and MoO from 500 °C to Mo/NC-800 catalysts (Fig. S4†). The Mo 3d5/2 XPS peak could
3
2
7
00 °C, and then to Mo
2
C beyond 775 °C. No PXRD peaks of be deconvoluted into two peaks with binding energies of 231.6
5
+
6+
MoO
3 2
or MoO were observed for the Co–Mo/NC-400 and and 232.2 eV, which are attributed to the Mo and Mo oxi-
Co–Mo/NC-600 catalysts, which was possibly due to the fact dation states for the Co–Mo/NC-400 catalyst (Fig. 3), respect-
2
3
6+
that MoO
of them was too small to be visible.
As shown in Fig. S1,† two peaks at around 1347 and 227.6 eV were observed for the Mo 3d5/2 XPS peak for the Co–
3
or MoO
2
did not form crystal structures or the size ively, and Mo was the main oxidation state, corresponding
to MoO . Peaks with binding energies at 231.1, 232.4 and
3
594 cm− were observed in the Raman spectra of the Co–Mo/ Mo/NC-600 catalyst, corresponding to the Mo , Mo oxi-
1
5+
6+
1
2
3
NC-T catalysts, corresponding to the D and G bands, respect- dation states and metallic Mo (Fig. S5†). Two peaks with
ively. The G band represents the graphitic structure of the cata- binding energies at 232.4 and 228.6 eV were the characteristic
2
0
6+
lysts, while the D band can be attributed to the lattice defects.
The intensity ratios of the D and G bands (I /I ) were calculated catalyst (Fig. S5†), and Mo
to be 1.005, 0.979 and 0.904 for the Co–Mo/NC-400, Co–Mo/ NC-800 catalyst, consistent with the PXRD results.
NC-600 and Co–Mo/NC-800 catalysts, and the decrease in the The hydrodeoxygenation of diphenyl sulfoxide into diphe-
Mo 3d5/2 XPS peaks of Mo and Mo
2
C of the Co–Mo/NC-800
D
G
2
C was the main state in the Co–Mo/
number of defects may result from the decrease in the number nyl sulfide was used as a model reaction to study the catalytic
of nitrogen atoms in the Co–Mo/NC-T catalysts as a result of the activity of the Co–Mo/NC-T catalysts (Table 1). Initially, the
21
increase in the reduction temperature.
reaction was conducted in the presence of a Co/NC-400 cata-
A transmission electron microscope (TEM) image (Fig. 2a) lyst, which was prepared by the same method as described for
revealed that the Co nanoparticles were homogeneously dis- the Co–Mo/NC-400 catalyst without the introduction of Mo. No
persed on the surface of the Co–Mo/NC-400 catalyst with an conversion of diphenyl sulfoxide was observed (Table 1,
average size of 10.1 nm in agreement with the XRD results. entry 1). In order to cleave the SvO bond, Mo as a common
However, the growth of cobalt nanoparticles into larger par- oxyphilic element was introduced into the nitrogen-doped
ticle size as well as aggregation were observed in the TEM carbon-supported Co catalysts to activate the SvO bond. As
images of the Co–Mo/NC-600 and Co–Mo/NC-800 catalysts. As expected, the Co–Mo/NC-400 catalyst could smoothly promote
shown in high-resolution TEM (HR-TEM) images (Fig. S2 and the hydrodeoxygenation of diphenyl sulfoxide into diphenyl
S3†), the lattice spacing measured for the crystalline plane was sulfide, affording a conversion of 77% after 2 h at 80 °C under
0
.20 nm (Fig. 2b), which corresponded to the d values of the 10 bar H
2
(Table 1, entry 2). However, the conversion of
(111) planes for metallic cobalt nanoparticles, while no lattices diphenyl sulfoxide sharply decreased to 24% over the Co–Mo/
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Fig. 3 XPS spectra of Co 2p and Mo 3d in the Co–Mo/NC-400 catalyst.
Table 1 Hydrodeoxygenation of diphenyl sulfoxide over different genation of sulfoxide. However, it showed no activity under the
a
catalysts
same reaction temperature of 80 °C (Table 1, entry 7). These
results suggested that the nitrogen atoms in the carbon
materials played important roles in the hydrodeoxygenation of
1
7,26
sulfoxides. According to previous reports,
nitrogen atoms
might play the following roles in the catalyst activity: (a) nitro-
Entry
Catalyst
Time (h)
Conversion (%)
Sel. (%) gen atoms act as Lewis base sites to promote the heterolytic
2
cleavage of H in coordination with the Co nanoparticles; (b)
1
2
3
4
5
6
7
Co/NC-400
2
2
2
2
2
2
2
0
77
24
0
0
0
—
Co–Mo/NC-400
Co–Mo/NC-600
Co–Mo/NC-800
100
100
—
—
—
the nitrogen atoms should have a strong interaction with Co
nanoparticles to improve the catalytic activity of Co nano-
particles and to stabilize them.
According to the above results, we can conclude that metal-
3
lic Co nanoparticles, nitrogen atoms and MoO played a syner-
MoO
3
Co–V/NC-400
Co–Mo/AC-400
0
—
a
gistic role in the hydrodeoxygenation of diphenyl sulfoxide.
Reaction conditions: Diphenyl sulfoxide (0.5 mmol), 1,4-dioxane
(10 bar), catalysts (20 mg), 80 °C.
(10 mL), H
2
MoO would activate the SvO groups by combination with the
3
oxygen atom in the SvO bond. Metallic Co nanoparticles acti-
vated hydrogen molecules with the assistance of nitrogen
−
+
+
NC-600 catalyst (Table 1, entry 3), and no conversion was atoms to generate the Co–H hybrid and N–H species. The H
+
observed over the Co–Mo/NC-800 catalyst (Table 1, entry 4). in N–H was transferred to the oxygen atoms in SvO bonds to
The decrease in the catalytic activity of the Co–Mo/NC-T cata- generate –S–OH species, and the sulfur–oxygen bond was then
lysts with an increase in the reduction temperature could be cleaved by H to give the target product.
−
mainly due to the following two reasons. On the one hand,
The reaction conditions were then optimized over the Co–
cobalt nanoparticles became larger and aggregated with an Mo/NC-400 catalyst. Firstly, the effect of the reaction solvents
increase of the reduction temperature, which provided fewer was studied. Generally, protic solvents with strong polarity as
active sites for the activation of H . On the other hand, the Mo well as water gave high to quantitative conversions of diphenyl
2
species in the Co–Mo/NC-T catalysts also played a crucial role sulfoxide (Table S1, entries 1 vs. 4†). Particularly, it is interest-
in the hydrodeoxygenation of diphenyl sulfoxide. As character- ing to note that the conversion of diphenyl sulfoxide in alco-
ized by XPS, MoO
NC-400 catalyst, while MoO
3
was the major component for the Co–Mo/ hols slightly increased in the order iso-PrOH, ethanol, and
was the major component for the methanol. Moderate conversions of diphenyl sulfoxide were
2
Co–Mo/NC-600 catalyst. MoO with abundant oxygen vacancy produced in N,N-dimethylformamide (DMF) and three other
3
sites (also called Lewis acid sites) was reported to be effective kinds of solvents with moderate polarity (Table S1, entries 5
2
4
in combining with oxygen atoms in the SvO bond, thus vs. 8†). The reaction in hexane with non-polarity produced the
facilitating the hydrodeoxygenation of diphenyl sulfoxide. This lowest conversion of diphenyl sulfoxide of 45%. The excellent
was further confirmed that in the Co–Mo/NC-800 catalyst was catalytic performance of the Co–Mo/NC-400 catalyst in water
completely inactive towards the hydrodeoxygenation of diphe- and alcohols could be for the following two reasons. On the one
nyl sulfoxide, in which Mo species was Mo C. A Co–V/NC-400 hand, the alcohol solvents and water should have the ability to
2
catalyst was also prepared with a similar method as for the activate the substrates or the intermediates via hydrogen-
Co–Mo/NV-400 catalyst, but it showed no catalytic activity for bonding interactions, similar to the hydrogenation of carbonyl
2
5
the hydrodeoxygenation of sulfoxides (Table 1, entry 6). These groups in ethanol. On the other hand, solvents with strong
results suggested that vanadium oxides could not activate the polarity resulted in a high dispersion of diphenyl sulfoxide and
substrates. We prepared an active carbon-supported Co–Mo the Co–Mo/NC-400 catalyst, as both of them were hydrophilic.
catalyst by a similar method, and used it in the hydrodeoxy- The best results were obtained in methanol, which produced
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a
diphenyl sulfide with a 100% yield at 80 °C and 10 bar H
2
. In Table 2 Substrate scope of the hydrodeoxygenation of sulfoxides
addition, the catalyst was active even at 25 °C and 10 bar, but it
took 60 h to get a high yield (Table S1, entry 10†).
The excellent catalytic performance of the Co–Mo/NC-400
catalyst at 80 °C inspired us to carry out the deoxygenation of
diphenyl sulfoxide under milder conditions. At a low tempera-
ture of 40 °C, it only produced 5% conversion of diphenyl sulf-
Entry Substrate
1
Temperature (°C) Time (h) Yield (%)
50
10
100
oxide after 2 h under 10 bar H (Fig. S6†). Then the conversion
2
greatly increased with an increase in the reaction temperature
from 50 to 80 °C. A quantitative conversion of diphenyl sulfox-
ide was observed at 80 °C after 2 h. Then the effect of hydrogen
pressure on the hydrodeoxygenation of diphenyl sulfoxide was
studied over the Co–Mo/NC-400 catalyst at 50 °C. As shown in
Fig. S7,† the conversion of diphenyl sulfoxide increased with
an increase of hydrogen pressure, especially from 1 to 10 bar.
For example, the conversion of diphenyl sulfoxide greatly
increased from 5% at 1 bar to 26% at 10 bar, and then slowly
increased to 33% at 20 bar. The increase in hydrogen pressure
resulted in an increase in the hydrogen concentration in the
reaction solution, resulting in an increase in the reaction rate.
Fig. S8† depicts the time course of the product distribution of
the hydrodeoxygenation of diphenyl sulfoxide over the Co–Mo/
2
3
50
50
15
18
100
100
4
50
20
100
5
6
7
50
50
50
24
22
26
100
100
100
NC-400 catalyst at 50 °C and 10 bar H . The percentage of
2
8
9
80
25
94
unreacted diphenyl sulfoxide gradually decreased during the
reaction process, and the yield of diphenyl sulfide gradually
increased on prolonging the reaction time. After 10 h, a full
conversion of diphenyl sulfoxide was obtained at 50 °C and 10
80
80
15
18
98
85
1
0
2
bar H with a 100% selectivity of diphenyl sulfide. It is worth
noting that the Co–Mo/NC-400 catalyst demonstrated compar-
a
Reaction conditions: Substrates (0.5 mmol), methanol (10 mL), H
2
(
10 bar), Co–Mo/NC-400 catalyst (20 mg).
able or even superior activity to the reported noble metal cata-
14
lysts. For example, a long reaction time of 24 h was required
to complete the conversion of diphenyl sulfoxide over a Pt–
1
4
Finally, recycling experiments of the Co–Mo/NC-400 catalyst
MoO
X
/TiO
2
catalyst at 50 °C under 7 bar H
2
,
while the reac-
were conducted. As shown in Fig. S9,† the catalyst showed
similar conversions of diphenyl sulfoxide of around 58%
during 6 runs, and the selectivity of diphenyl sulfide remained
at 100%. These results suggested that the Co–Mo/NC-400 cata-
lyst showed a high stability. ICP-AES analysis of the solution
after the first reaction showed that the Co and Mo contents in
the solution were below the detection limits.
In conclusion, we have developed a new method for the use
of heterogeneous non-noble metal catalysts for the catalytic
hydrogenation of sulfoxides for the first time. Notably, the as-
prepared Co–Mo/NC-400 catalyst exhibited an unprecedented
activity under mild reaction conditions (25–80 °C and 10 bar
tion time to get full conversion of diphenyl sulfoxide was only
1
2 h over the Co–Mo/NC-400 catalyst at 50 °C under 10 bar H2.
Then, the scope of the as-prepared Co–Mo/NC-400 catalyst
towards the hydrodeoxygenation of various sulfoxides was
investigated. A number of aromatic, benzylic and aliphatic sulf-
oxides were smoothly converted into the corresponding sulfides
with excellent yields (Table 2, entries 2–6). Compared with
diphenyl sulfoxide, substituted diphenyl sulfoxides with large
steric hindrance were less active, which required a longer reac-
tion time to achieve high conversions (Table 2, entries 1–4).
Meanwhile, a substituted diphenyl sulfoxide with electron-with-
drawing groups showed lower activity than counterparts
2
H ) and greatly broadens the applicability to a wide range of
(Table 2, entry 2 vs. entries 3 and 4). Phenylalkyl sulfoxides
sulfoxides. Control experiments demonstrate that metallic Co
and Mo in the Co–Mo/NC-400 catalyst played a synergistic role
in the hydrogenation of sulfoxides with H . The activity of the
2
as-prepared Co–Mo/NC-400 catalyst was comparable to or even
demonstrated similar activity to diphenyl sulfoxides (Table 2,
entry 2 vs. entries 3 and 4), which have small steric hindrance
but low electronic density of the sulfoxide groups. Dialkyl sulf-
oxides were also successfully converted into the corresponding
alkyl sulfides, which were performed at high reaction tempera-
ture of 80 °C, due to the low activity of the dialkyl sulfoxides
with low electron density of the sulfoxide groups. Notably, the
Co–Mo/NC-400 catalyst was effective for the selective dehydro-
genation of sulfoxide substrates without affecting other reduci-
ble functional groups such as CvC double bonds, Cl, and Br.
higher than that of reported noble metal catalysts.
Conflicts of interest
There are no conflicts to declare.
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1
2 K. Ogura, M. Yamashita and G. Tsuchihashi, Synthesis,
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