Thiolation of dimethyl sulfide to methanethiol over WO3/ZrO 2 catalysts

Article abstract of DOI:10.1016/j.molcata.2012.08.009

The thiolation of dimethyl sulfide with H₂S over a variety of tungsten-zirconia (WO₃/ZrO₂) catalysts with different contents of WO₃ was studied. The maximum yield of methanethiol was obtained at the reaction temperature of 633 K in the presence of 10 wt.%WO ₃/ZrO₂ catalyst. XRD, BET and TPD characterization results reveal that supporting WO₃ species on ZrO₂ gives rise to the improvement of both in structure stability and surface acidity. The MT yield increased first and then decreased with the increase of reaction temperature for all of the catalysts due to the decomposition of methanethiol and dimethyl sulfide. The optimum loading of WO₃ was found to be 5-10 wt.% (with a surface density of 3.5-4.5 w-atom nm^-2). Furthermore, the WO ₃/ZrO₂ catalyst exhibits high resistance to water.

Full text of DOI:10.1016/j.molcata.2012.08.009

Journal of Molecular Catalysis A: Chemical 365 (2012) 60–65
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Thiolation of dimethyl sulfide to methanethiol over WO3/ZrO2 catalysts
Shiping Chena, Weiming Wanga, Yuanhua Zhanga, Yucai Weib, Weiping Fanga, Yiquan Yangb,∗
a Department of Chemical and Biochemical Engineering, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, PR China
b Department of Chemistry, College of Chemistry and Chemical Engineering, and National Engineering Laboratory for Green Chemical Productions of Alcohols, Ethers and Esters,
Xiamen University, Xiamen 361005, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 July 2012
Received in revised form 12 August 2012
Accepted 13 August 2012
Available online 20 August 2012
The thiolation of dimethyl sulfide with H2S over a variety of tungsten–zirconia (WO3/ZrO2) catalysts with
different contents of WO3 was studied. The maximum yield of methanethiol was obtained at the reaction
temperature of 633 K in the presence of 10 wt.%WO3/ZrO2 catalyst. XRD, BET and TPD characterization
results reveal that supporting WO3 species on ZrO2 gives rise to the improvement of both in structure
stability and surface acidity. The MT yield increased first and then decreased with the increase of reaction
temperature for all of the catalysts due to the decomposition of methanethiol and dimethyl sulfide. The
optimum loading of WO3 was found to be 5–10 wt.% (with a surface density of 3.5–4.5 w-atom nm−2).
Furthermore, the WO3/ZrO2 catalyst exhibits high resistance to water.
Keywords:
Dimethyl sulfide
Methanethiol
Thiolation
© 2012 Elsevier B.V. All rights reserved.
WO3/ZrO2
1. Introduction
dimethyl sulfide, in which the content of water can achieve 5%,
thereby loses its activity because of its hydrophilic nature.
Methanethiol (CH3SH, MT) is an important chemical inter-
medium widely used in the production of pesticides, pharmaceut-
icals, petrochemicals, and especial for methionine synthesis [1,2].
During the last few years, with increasing stringently in the demand
for methionine, there is a growing demand for methanethiol in
order to meet the requirements [3,4]. The synthesis of methanethiol
has received global attention, by now; several routes for its gener-
ation have been explored [5–12].
Industrially, methanethiol is mainly synthesized by thiolation of
methanol with H2S, accompanying with the formation of dimethyl
sulfide (CH3SCH3, DMS), a major byproduct [5]. However, dimethyl
sulfide was incinerated as a wastegas generally. Beach [7] pro-
posed first to recover dimethyl sulfide and used it as a feed for
methanethiol synthesis, which performs in an independent by-pass
reactor, during the synthesis of CH3SH from CH3OH and H2S, in such
a way that the byproduct DMS can be converted to more valuable
product MT. This is economically as well as environmentally attrac-
tive for better carbon management. The catalysts for this reaction
were solid acid catalysts like ␥-Al2O3 and P promoted Al2O3 [8,9].
The ␥-Al2O3 catalyst is active; nevertheless, the major limitation
for the usage of ␥-Al2O3 is the existence of water in the recovered
CH3SCH3 + H2S → 2CH3SH
(1)
WO3/ZrO2 catalysts have received much attention in the recent
catalysis field [13–16]. Hino and Arata [17] found that tungsten
oxide-supported zirconia possesses appropriate superacid sites
and smaller pore volume in comparison with other acid catalysts
such as alumina or zeolite, and its application leads to effective
downsizing of the reaction unit. Furthermore, the catalysts are
extremely stable at high temperature in H2, O2 or H2O atmospheres
[18,19]. Therefore, it is promising one to be used as a novel catalyst
for the reaction of DMS with H2S.
In the present work, we aimed to study the thiolation of DMS
in the presence of pure ZrO2 and WO3/ZrO2 catalysts. The effect
of catalyst preparation conditions, i.e. calcination temperature and
tungsten loading on the catalyst performance was intensively stud-
ied. In addition, the effect of water content in the system on the
reaction performance of the catalysts was explored as well.
2. Experimental
2.1. Catalyst preparation
2.1.1. Preparation of the ZrO2 support
ZrO2 was prepared by the reverse precipitation method.
Appropriate amount of 0.1 M aqueous solution of zirconyl chlo-
ride (ZrOCl2·8H2O, Sinopharm Chemical Reagent Co., Ltd.) was
added dropwise to ammonium hydroxide solution (NH4OH, 25%,
∗
Corresponding author. Tel.: +86 592 2180361; fax: +86 592 2180361.
E-mail address: yyiquan@xmu.edu.cn (Y. Yang).
1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2012.08.009

S. Chen et al. / Journal of Molecular Catalysis A: Chemical 365 (2012) 60–65
61
Sinopharm Chemical Reagent Co., Ltd.) at room temperature under
well-stirring. The pH of the solution was controlled at 9, when a
buoyant white powder immediately produced. After stirring for
6 h at room temperature, the precipitate was evaporated in oil
bath at 373 K, and then washed with deionized water five times,
following by drying overnight at 383 K to generate a sample Zir-
conium hydroxide Zr(OH)4. Finally, the sample was devised into
several parts and calcined respectively at different temperatures
(773, 873, 973, 1073 and 1173 K) for 3 h to get ZrO2 samples. The
samples are designated as ZrO2773, ZrO2873, ZrO2973, ZrO21073
and ZrO21173.
The actual content of tungsten oxide in xW/ZrO2 samples were
determined by a Bruker S8 TIGER X-ray fluorescence (XRF) spec-
trometer.
NH3 and CO2 temperature-programmed desorption (TPD) were
performed in a quartz tube reactor filled with 80 mg catalyst. For
NH3-TPD experiment, the catalyst was pretreated in Ar at 673 K for
1 h, then cooled to 323 K, at which ammonia was adsorbed for about
0.5 h in an ammonia stream of 30 mL min−1. Loosely bound NH3 was
removed by Ar sweeping at 323 K, and then the temperature was
increased to 973 K at a heating rate of 10 K min−1. The desorbed
NH3 was monitored with a MS signal of m/e = 16 and 17 in multi-
ple ion detection (MID) mode. So did the CO2-TPD experiment as
NH3-TPD with CO2 substituting for NH3. The CO2-TPD experiment
was conducted from 323 to 873 K. Desorbed CO2 component was
monitored with a MS signal of m/e = 44 in MID mode.
2.1.2. Preparation of the WO3/ZrO2 catalysts
The WO3/ZrO2 catalysts with various weight percentages
(2.5, 5.0, 7.5 and 10%) were prepared by the incipient wetness
impregnation in ammonium metatungstate solution (precur-
sor (NH4)10W12O41·5H2O, 98% purity). The Zirconium hydroxide
Zr(OH)4 sample was impregnated in the aqueous solution of
calculated ammonium metatungstate. Then, the sample was
dried overnight at 383 K and calcined at 973 K for 3 h. The
catalyst with x wt.% of WO3 was designated as xW/ZrO2; for
example, 10 wt.%WO3 supported on ZrO2973 is expressed as
10 W/ZrO2973.
3. Results and discussion
3.1. Structural properties
The main physicochemical properties of the tungsten–
zirconium mixed oxides are presented in Table 1. The surface
density of W species (w-atom nm−2) was calculated from Eq. (2)
[15,20]. Increasing the calcination temperature of WO3-free zirco-
nia led to a continuous decrease in the surface area owing to the
sintering and crystallizing of amorphous zirconia support and sig-
nificant pore collapse [14,16]; however, the surface area increased
as increasing the WO3 loading at 973 K, which can be attributed to
the decrease of ZrO2 sintering rate caused by WO3 adding, mean-
while, the high resistance against ZrO2 sintering tends to getting
finer pores.
2.2. Reaction conditions
DMS conversion reaction was conducted in a glass tubular fixed
bed reactor with an internal diameter of 10 mm typically, 2.0 mL of
the catalyst with 20–40 mesh was filled into the reactor, with a thin
layer of glass fiber and a layer of quartz powder (20–40 mesh) cov-
ered on the catalyst. Before experiment, the catalyst was sulfurized
with H2S for 1 h at 673 K for activating; the H2S flow rate was main-
tained by mass flow controller (Beijing Seven star, D08-1F). Then
the sulfurized catalyst was tested at different temperatures (553,
573, 593, 613, 633, 653, 673 and 693 K) in turn, and the system pres-
sure was held at 0.5 MPa with the aid of a back-pressure regulator.
The DMS solution was injected into the catalyst bed by a precision
metering pump (Beijing Satellite Manufactory, 2ZB-1L10). The out-
let stream temperature was kept at 400 K with heater band, and the
effluent was analyzed by an on-line gas-chromatograph equipped
with a Porapak Q (Ф2 m × 3 mm) column connected to a thermal
conductivity detector (TCD).
WC × (6.02 × 1023
)
SW
=
(2)
100 × FWWO × (SBET × 1018
)
3
where SW: W surface density (w-atom nm−2), WC: WO3 content
(wt.%), FWWO : formula weight of WO3 (231.8 g mol−1), SBET: sur-
3
face area (m2 g−1).
The XRD patterns arising from ZrO2 catalysts calcined at the
temperatures from 773 to 1173 K and the WO3/ZrO2 catalysts with
different WO3 concentrations are shown in Fig. 1(A) and (B). It
can be seen that the crystallinity of ZrO2 in monoclinic phase
increased with increasing the support calcination temperature.
When WO3 was loaded onto ZrO2, the ratio of monoclinic phase to
tetragonal phase decreased with increasing WO3 loading at 973 K,
providing further evidence of the stabilizing effect of surface WO3.
These changes in the phase-transformation observed according to
support calcination temperature and tungsten oxide loading are
consistent with those reported in the literature [21–24]. No crys-
talline WO3 phase (2ꢁ = 23.2◦, 23.7◦, and 24.3◦) can be detected
even if the WO3 content being as high as 10 wt.%, indicating that
tungsten oxide was present in a highly dispersed manner. The SEM
micrographs (Fig. 2) for ZrO2 and WO3/ZrO2 catalysts show similar
quasi spherical morphological feature and the WO3/ZrO2 catalysts
exhibit better uniformity, suggesting that tungsten species over
throughout zirconia were well dispersed and could not crystallize
separately.
To study the effect of H2O on the activity of the catalyst for
the reaction, DMS with different contents of water, which was
incorporated into DMS in advance by using a precision meter-
ing pump (Beijing Satellite Manufactory, 2ZB-1L10), was used as
feedstock.
2.3. Catalyst characterization
XRD measurements were performed on a Panalytical X’pert
PRO X-ray diffractometer utilizing monochromatic Cu K␣ radia-
tion (ꢀ = 0.15418 nm, tube voltage: 40 kV, tube current: 30 mA) in
the 2ꢁ range from 10◦ to 70◦. The morphology of the catalyst was
observed on field emission scanning electron microscopy (FE-SEM,
LEO1530).
X-ray photoelectron spectroscopy (XPS) was performed on a
Phi Quantera spectrometer equipped with an aluminum anode
(AlK␣ = 1486.6 eV) operated at 25 W (15 kV, 1.67 mA).
The surface areas of the catalysts were measured using nitro-
gen adsorption at 77 K with a Micromeritics Tristar 3000 surface
area and pore analyzer. Prior to N2 physisorption measurement, all
samples were degassed at 393 K for 1 h and then evacuated at 573 K
for 3 h to remove physically adsorbed impurities.
3.2. Surface acid–base properties
The intensities and quantities of acidic and basic sites on the
catalysts were measured by NH3-TPD and CO2-TPD techniques,
respectively; the experiment results obtained are presented in
Fig. 3. In contrast with pure ZrO2, the four WO3/ZrO2 catalysts
mainly exhibit two peaks on the NH3-TPD curves. The low-
temperature peak is assigned to ammonia desorbed from the

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S. Chen et al. / Journal of Molecular Catalysis A: Chemical 365 (2012) 60–65
Table 1
Physicochemical properties of tungsten–zirconium mixed oxides.
Catalysts
WO3 content (wt.%)
Surface area (m2 g−1
)
Pore volume (cm3 g−1
)
Pore diameter (nm)
W-surface density (w-atom nm−2
)
ZrO2773
ZrO2873
ZrO2973
ZrO21073
–
–
–
–
68.2
39.5
20.6
12.9
10.4
29.7
32.9
38.9
43.1
53.0
76.5
0.17
0.17
0.14
0.07
0.10
0.18
0.15
0.17
0.18
0.17
0.21
3.7
6.4
11.9
12.9
20.8
9.8
8.0
8.1
7.5
8.9
–
–
–
–
ZrO21173
–
–
2.5 W/ZrO2973
5 W/ZrO2973
7.5 W/ZrO2973
10 W/ZrO2973
15 W/ZrO2973
20 W/ZrO2973
2.17
4.19
5.75
7.15
9.49
13.95
1.90
3.31
3.84
4.31
4.65
4.74
8.7
Fig. 1. XRD patterns due to the ZrO2 (A) and WO3/ZrO2 catalysts (B).
Fig. 2. SEM images of the catalysts. (a) ZrO2973, (b) 2.5 W/ZrO2973, and (c) 10 W/ZrO2973.
weaker acid sites centered at around 400 K, and the shoulder peak is
due to ammonia desorbed from the stronger acid sites between 500
and 650 K. The NH3-TPD spectra revealed that as the WO3 content
increased, the areas of the TPD spectra increased, suggesting that
the doping amount of tungsten oxide in the range of 2.5–10 wt.%
was responsible for the acidity and determined the quantities of
surface acidic sites. On the other hand, the CO2 desorption occurred
at the relatively low temperatures ranging from 330 to 390 K, which
can be assigned to weak basic sites for all of the catalysts. Notably,
the area below the curves decreased as the WO3 content increased,
which indicates that the surface basicity decreased due to the load-
ing of tungsten oxide, the decrease trend is particularly significant
for the 10 W/ZrO2973.
3.3. Performance of pure WO3 and ZrO2
The reaction of H2S and DMS was carried out at 593 K over
pure WO3 and ZrO2 calcined at different temperatures. The initial
activities and the activities after 3 h reaction are summarized in
Fig. 4. The initial activity of pure WO3 was found to be maximal
among these catalysts, but remarkable deactivation occurred in 3 h
time on stream. Calcining at lower temperatures (773–973 K) made
all of the ZrO2 catalysts showed a moderate conversion of DMS
at about 15%; however, the conversion of DMS decreases for the
catalysts calcined at higher temperatures (1073–1173 K), this phe-
nomenon can be due to the decrease in the catalyst surface area.
Fortunately, the deactivation did not occur on the ZrO2 series in 3 h

S. Chen et al. / Journal of Molecular Catalysis A: Chemical 365 (2012) 60–65
63
Fig. 3. TPD spectra of WO3/ZrO2 catalysts. (a) NH3-TPD and (b) CO2-TPD.
time on stream. The formation of coke was confirmed to be the rea-
son for the deactivation, the observation on the quick deactivation
of pure WO3 may be attributed to coke deposition which cannot be
removed immediately from the surface [25,26]. According to above
results, a series of tungsten oxide catalysts with different tungsten
contents loaded on ZrO2973 were investigated hereafter in detail.
for the strong decrease of the selectivity toward MT. This suggests
that the catalysts with high content of WO3 own more acid sites,
which contributed to violent decomposition of dimethyl sulfide
and methanethiol, and decreased MT yield rapidly. We tested the
15 W/ZrO2973 and 20 W/ZrO2973 samples at 633 K, and found that
they got a DMS conversion close to 10 W/ZrO2973, but the MT selec-
tivity of these two catalysts were 88% and 84%, respectively. So, the
optimum loading of WO3 was confirmed to be 5–10 wt.%(with a
surface density of 3.5–4.5 w-atom nm−2).
3.4. Catalytic activity of WO3/ZrO2
Fig. 5 shows the effect of W contents on the performance of
WO3/ZrO2 catalysts. The 10 W/ZrO2973 catalyst exhibits the high-
est conversion of DMS, while the MT yield increased first and then
decreased for all of the catalysts. The thiolation of DMS to MT is
a simple reaction at relative low temperature with unique prod-
uct MT [8,9,27,28], further increasing in reaction temperature is
expected to aggravate the decomposition of DMS and MT them-
selves, generating a series of low hydrocarbon compounds (CH4,
C2H6), element sulfur and carbon, as a result, lowering the MT
yield. The tungsten oxide-supported ZrO2 catalysts exhibit higher
conversion of DMS and yield of MT at relative lower reaction tem-
perature in comparison with pure ZrO2973 sample, but when the
reaction temperature rose up to 653 K, the MT yield decreased fast
Fig. 6 shows XPS spectra due to the WO3/ZrO2 catalyst. Based
on the XPS studies, most tungsten oxidation states in WO3/ZrO2
were found to be W6+, W5+and W4+ [29–31]. After reaction, the W6+
and W5+ cations totally transformed into W4+ cation, suggesting
that the W4+ species participated in thiolation of DMS. Compared
the curve (c) with (a), We found that after recalcined in air, the
amount of W6+ cation seems to increase in contrast to the sample
before reaction, meanwhile, the peak area of Zr4+ increases, sug-
gesting that the recalcination may weaken the interaction between
ZrO2 and WO3. However, the activity did not change, so we use the
recalcined sample in the next water-adding test.
Fig. 5. Catalytic performance of the WO3/ZrO2 catalysts for the thiolation of DMS
with H2S to MT. Reaction conditions: GHSV = 1000−1, P = 0.5 MPa, H2S/DMS = 4.0, and
catalyst: 2 mL.
Fig. 4. Activity at 593 K over pure WO3 and ZrO2 catalysts. Reaction conditions:
GHSV = 1000−1, P = 0.5 MPa, H2S/DMS = 4.0, and catalyst: 2 mL.

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S. Chen et al. / Journal of Molecular Catalysis A: Chemical 365 (2012) 60–65
Fig. 6. XPS spectra of Zr 4p and W 4f region for 5 W/ZrO2973. (a) Fresh 5 W/ZrO2973, (b) after reaction at 593 K for 3 h, (c) after reaction at 593 K for 3 h, then calcined at
973 K in air for 3 h.
3.5. Effect of water content on the thiolation of DMS to MT over
5 W/ZrO2973 and 10 W/ZrO2973
WO3/ZrO2 have excellent surface hydrophobicity (the reciprocal of
the density of water adsorbed). According to the reaction of H2S
and DMS, the hydrophobic nature of the WO3/ZrO2 catalyst may
reasonably guarantee the access of dimethyl sulfide to active sites
and suppress the poisoning effect of water.
It is well established that water present in feedstock exerts neg-
ative effect on the acid-catalyzed reactions [22,32]. In the ␥-Al2O3
system, the presence of water during the thiolation reaction results
in hydrolysis reaction, which eventually reduces the MT yield and
speeds up the deactivation. Hence, in this study, the effect of water
content (0–10%) added on the thiolation of DMS to MT was investi-
gated at 593 K. The results are shown in Fig. 7. Fig. 7 indicates that
the presence of water indeed lowered the yield of MT, especially
when the water content is more than 5%, although the decreasing
proportion is acceptable. Chu et al. [15] have demonstrated that
3.6. Effect of reaction time on the thiolation of DMS to MT
Fig. 8 shows the effect of reaction time on the DMS conversion
and MT selectivity catalyzed by 5 W/ZrO2973 sample at 633 K. It can
be seen from the plot that a plateau of DMS conversion is attained in
a short time of reaction (1 h); the DMS conversion does not increase
much with further increase in the reaction time even up to 12 h. On
the other hand, the MT selectivity maintained at a high and stable
level for the whole 12 h.
Fig. 7. Effect of water content on the MT yield. (ꢀ) 5 W/ZrO2973, (᭹) 10 W/ZrO2973.
Fig. 8. Catalytic performance for the 5 W/ZrO2973 catalyst as a function of
Reaction conditions: reaction temperature = 593 K, GHSV = 1000−1
H2S/DMS = 4.0, and catalyst: 2 mL.
,
P = 0.5 MPa,
time on stream. Reaction conditions: reaction temperature = 633 K, GHSV = 1000−1
,
P = 0.5 MPa, H2S/DMS = 4.0, and catalyst: 2 mL.

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65
4. Conclusions
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Supporting WO3 species on ZrO2 gives rise to the improvements
of both in structure stability and surface acidity. The activity of
the series of WO3/ZrO2 catalysts increased remarkably depending
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then decreased with the increase of reaction temperature espe-
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