Investigation on converting 1-butene and ethylene into propene: Via metathesis reaction over W-based catalysts

Article abstract of DOI:10.1039/c7ra13776a

Supported W catalysts were extensively investigated for the conversion of 1-butene and ethylene into propene by metathesis reaction. The performance of catalysts was compared by using unsupported WO₃, pure SBA-15, supported W/SBA-15 with different W loadings, varied calcination temperatures, and by changing the pretreatment gas atmosphere. The above catalytic results could be employed to deduce the reaction mechanism combined with characterization techniques such as BET, XRD, UV-vis DRS, Raman, pyridine-IR, XPS, and H₂-TPR. In this study, over the investigated W/SBA-15 catalysts, the results showed that the silanol group (Si-OH) in SBA-15 could act as a weak Br?nsted acid site for 1-butene isomerization. However, the metathesis reaction was catalyzed by W-carbene species. The initially formed W-carbenes (WCH-CH₃) as active sites were derived from the partially reduced isolated tetrahedral WO_x species which contained WO or W-OH bonds in W⁵⁺ species as corresponding Lewis or Br?nsted acid sites. Furthermore, the W/SBA-15 being pretreated by H₂O led to a complete loss of the metathesis activity. This was mainly due to the sintering of isolated WO_x species to form an inactive crystalline WO₃ phase as demonstrated by XRD patterns. On the other hand, the reduction of WO_x species remarkably suppressed by H₂O pretreatment was also responsible for the metathesis deactivation. This study provides molecular level mechanisms for the several steps involved in the propene production, including 1-butene isomerization, W-carbene formation, and metathesis reaction.

Full text of DOI:10.1039/c7ra13776a

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Investigation on converting 1-butene and ethylene
into propene via metathesis reaction over W-based
catalysts†
Cite this: RSC Adv., 2018, 8, 8372
Guangzheng Zuo, Yuebing Xu, Jiao Zheng, Feng Jiang and Xiaohao Liu
*
Supported W catalysts were extensively investigated for the conversion of 1-butene and ethylene into
propene by metathesis reaction. The performance of catalysts was compared by using unsupported
3
WO , pure SBA-15, supported W/SBA-15 with different W loadings, varied calcination temperatures, and
by changing the pretreatment gas atmosphere. The above catalytic results could be employed to deduce
the reaction mechanism combined with characterization techniques such as BET, XRD, UV-vis DRS,
2
Raman, pyridine-IR, XPS, and H -TPR. In this study, over the investigated W/SBA-15 catalysts, the results
showed that the silanol group (Si–OH) in SBA-15 could act as a weak Brønsted acid site for 1-butene
isomerization. However, the metathesis reaction was catalyzed by W-carbene species. The initially
formed W-carbenes (W]CH–CH
tetrahedral WO species which contained W]O or W–OH bonds in W species as corresponding Lewis
or Brønsted acid sites. Furthermore, the W/SBA-15 being pretreated by H O led to a complete loss of
the metathesis activity. This was mainly due to the sintering of isolated WO species to form an inactive
crystalline WO phase as demonstrated by XRD patterns. On the other hand, the reduction of WO
species remarkably suppressed by pretreatment was also responsible for the metathesis
3
) as active sites were derived from the partially reduced isolated
5+
x
2
x
Received 31st December 2017
Accepted 16th February 2018
3
x
2
H O
DOI: 10.1039/c7ra13776a
deactivation. This study provides molecular level mechanisms for the several steps involved in the
propene production, including 1-butene isomerization, W-carbene formation, and metathesis reaction.
rsc.li/rsc-advances
route can provide a reliable solution for meeting the demand for
propene by utilizing the abovementioned alternative carbon-
1
. Introduction
12
Propene is one of the fastest growing raw materials as a key containing resources via syngas conversion.
building block in the chemical industry, driven primarily by the
Compared with the highly selective production of propene
high growth rate of polypropene consumption. Also, propene via PDH route, the FTO route generally produces the mixed light
1,2
13,14
is used in the production of propene oxide which is a chemical a-olens such as ethylene, propene, and 1-butene.
In spite of
3
precursor for the synthesis of propene glycol and polyols. The the high selectivity to total light a-olens, the selectivity to
rest of propene is consumed for the synthesis of acrylonitrile, propene is usually lower than 40%. Therefore, it is necessary for
oxo-alcohol, cumene, and many other industrially relevant us to search for an efficient approach to signicantly enhance
4
chemicals. The market for propene has been continuously the propene fraction in light a-olens in order to meet its
growing at average rates of 4–5% per year. More than 50 million growing demand. Olen metathesis is a highly versatile process
tons of additional capacity will be needed by the year of 2020. for interconverting olenic hydrocarbons. It is an efficient
Currently, approximately 42% of propene is primarily derived approach for us to remarkably enhance the selectivity to pro-
from steam cracking of naphtha and about 39% of it is recov- pene by using the metathesis reaction of 1-butene and ethylene
5–8
ered from uid catalytic cracking (FCC). In recent years, from FTO route.
9
dehydrogenation of propane (PDH), methanol-to-olens
(
Various supported transition metal oxides have been used as
10
11
MTO), and Fischer–Tropsch to olens (FTO) processes for heterogeneous catalysts for metathesis reaction, including
15–17 18,19 20,21
the production of propene have been blooming in view of the tungsten (W),
molybdenum (Mo),
and rhenium (Re).
availability of cheap propane in shale gas and the availability of Among these catalysts, supported W catalysts obtain the largest
coal-, natural gas-, and biomass-derived syngas. Especially, FTO number of commercial applications because of its benets such
as relatively lower price, better stability, and better resistance to
22
poisoning. Despite that numerous literatures have reported
the olen metathesis by the supported W catalysts, most of
them focus on conversion of 2-butene and ethylene into pro-
Department of Chemical Engineering, School of Chemical and Material Engineering,
Jiangnan University, Wuxi 214122, China. E-mail: liuxh@jiangnan.edu.cn
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c7ra13776a
23–25
26,27
pene,
and its reverse process.
Self-metathesis of 1-
1
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2
8–30
31
32
butene
or 2-butene, 2-pentene metathesis, and metath- formation, and metathesis reaction on SBA-15 supported W
33–35
esis of 1-butene and ethylene
have also been paid attention catalysts have been proposed and discussed in detail.
to. However, for converting FTO-based a-olen products such 1-
butene and ethylene into propene by metathesis reaction, the
detailed investigation on the reaction mechanism is still
insufficient over past studies. Therefore, the molecular level
2
. Experimental
2.1 Catalyst preparation
insights into the nature of surface active tungsten oxide (WO
sites, catalyst activation, and reaction mechanism still need to wetness impregnation (IWI) method as described in literature.
be taken in order to guide the rational design of advanced The SBA-15 support was purchased from Jiaxing Tanli New
catalysts.
Materials Development Co., Ltd (China). Ammonium meta-
It is generally accepted that the high dispersion of WO and tungstate (AMT, (NH ) H W O $xH O, 99.5%, Aladdin) was
x
)
The supported tungsten catalysts were prepared by an incipient
37
x
4 6
2
12 40
2
its existing state on support surface play crucial roles in the used as tungsten precursor. Briey, prior to impregnation, SBA-
ꢀ
catalyst efficiency. Especially, the isolated WO
x
species might be 15 was dried in an oven at 120 C for 2 h. Then, as-calculated
36
responsible for metathesis activity. And these factors depend amount of AMT aqueous solution was added into the dried
on the properties of supports and the preparation methods. SBA-15. Aer impregnation, the sample was dried at 120
ꢀ
C
ꢀ
Therefore, we have designed a series of supported W catalysts overnight and followed by calcination in air at 550 C for 4 h
ꢀ
ꢁ1
with different physicochemical properties and varied prepara- with a heating rate of 1 C min . The obtained catalysts were
tion conditions to regulate the WO species state. In this study, labelled as xW/SBA-15 where x represents WO3 content in
x
several supports such as SiO , g-Al O , TiO , and SBA-15 are weight percentage. The supported W catalysts with different
2
2
3
2
selected for the preparation of W catalysts, and unsupported supports such as SiO , g-Al O , and anatase-TiO were prepared
2
2
3
2
3
3
WO catalyst is also prepared as a reference. The as-prepared W by the same method. In addition, the unsupported WO used as
catalysts and pure support are used for the metathesis reaction a reference was prepared by calcination of AMT with same
of 1-butene and ethylene; this can provide important informa- procedures for supported catalysts.
tion for speculation on what species in catalysts contribute to
the 1-butene isomerization and the olen metathesis for the
formation of propene. Furthermore, the calcination tempera-
ture and W loading in catalyst may affect the aggregation of
WOx species to lead to the changing ratio of isolated WOx
2
.2 Catalyst characterization
Brunauer–Emmett–Teller (BET) surface area, pore volume, pore
size, and the adsorption–desorption isotherms were measured
by using a Micromeritics ASAP 2020. The X-ray diffraction (XRD)
patterns of samples were recorded on a Bruker AXS D8 Advance
X-ray diffractometer using Cu (Ka) radiation with 0.02 step in
the 2q range from 10 to 80 . Diffuse-reectance UV-vis spectra
species to crystalline WO
the high dispersion of WO
In addition, prior to metathesis reaction, the different
pretreatment gas, such as the oxidizing gas (O and H O),
reducing gas (H ), and inert gas (N ), can regulate the initially
3
, which can conrm the importance of
x
phase.
ꢀ
ꢀ
2
2
(UV-vis DRS) were obtained in the range of 200–700 nm with
2
2
a Shimadzu UV-3600 Plus Spectrometer, using BaSO as refer-
ence. The Raman spectra of the samples were recorded on
a Renishaw Micro Raman Spectrometer in the range from 200 to
4
existing state of WOx species. The induction period for the
formation of propene over gas-pretreated supported W catalysts
could be helpful for us to discern that either oxidative or
ꢁ
1
1200 cm (excitation line: 785 nm of diode solid-state laser).
reductive state of WO
reaction. It should be noted that H
x
species is responsible for metathesis
O is used to pretreat the
The XPS data were collected using X-ray photoelectron spec-
trometer Thermo SCIENTIFIC ESCALAB 250xi with an Al-Ka
2
supported W catalysts before reaction, which is based on the
following considerations. On the one hand, the pretreatment
with H O might result in the oxidation of WO species; on the
2 x
(1486.8 eV) X-ray source. The pyridine-absorption infrared
spectra (pyridine-IR) were recorded on a Thermo Nicolet 5700
FT-IR Spectrometer. Temperature programmed reduction of
hydrogen (H
other hand, the pretreatment with H O might lead to more
2
2
-TPR) were recorded on a Xianquan TP5076 auto-
crystalline WO3 phase due to the sintering. The designed
pretreatment methods might make obvious transformation
mated chemisorption analyzer equipped with a TCD detector.
between oxidative and reductive WO
lated and crystalline WO phases. The metathesis activity over
O-pretreated W catalysts could provide further verication on The catalytic experiments of W-based catalysts for metathesis of
x
species, or between iso-
2.3 Catalytic evaluation
x
H
2
the proposed mechanism as reected by abovementioned ethylene and 1-butene to propene were carried out in a xed bed
a series of investigations. In order to accurately correlate the reactor. 0.5 g of shaped catalyst (40–60 mesh) was placed at the
relationship between the structure of supported W catalysts and center of reactor. The W-based catalysts need to be pretreated at
ꢀ
catalytic performance, various characterization methods such 550 C and 1 bar for 4 h by pure N or other gases such as pure
2
as BET, XRD, UV-vis DRS, Raman, pyridine-IR, XPS, and H -TPR H , 1% O /N , and water vapor in N . It should be noted that the
2
2
2
2
2
ꢁ
1
have been applied to measuring the physicochemical properties ow rate of pretreated gas was 35 mL min . Water vapor
of catalysts. Finally, the inuences of WHSV and reaction generated in a ask at 80 C was introduced into reactor with
temperature on metathesis reaction are also investigated to a owing N2. In the case of water pretreatment, the catalyst
obtain the optimal catalytic activity and propene selectivity. And was further ushed with inert N
ꢀ
38
ꢀ
2
at high temperature of 450 C
the mechanisms for 1-butene isomerization, W-carbene for 3 h in order to avoid the inuence of the remaining presence
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of H
2
O prior to the reaction. Aer treatment and cooling to abovementioned WO
3
, which might be ascribed to the W highly
ꢀ
designed reaction temperature, for example 450 C, the mixed dispersed on SBA-15 and its strong interaction with g-Al O ,
2
3
40
feed gas (molar ratio ethylene/1-butene ¼ 2/1) was fed into the respectively. The W loaded on TiO with small surface area
2
catalyst bed. The gas weight hourly space velocity (WHSV), leads to a smaller amount of the crystalline WO3 formed
dened as the weight ratio of mixed gas to catalyst packed, was compared to that for SiO
2
support, which might result from
regulated at 1.8 h . The ow rates of ethylene and 1-butene more favorable wetting of TiO by WO than that on SiO . For
were controlled by two mass ow controllers (MFC, Brooks, comparison, the prepared unsupported WO reference exhibits
model 5800E). All the products were analyzed online by a gas the intensive diffraction peaks indicating the presence of WO
chromatograph (7820A, Agilent) with a HP-plot/Al KCl (Fig. S1c†).
column using a ame ionization detector (FID). The 1-butene The UV-vis DRS spectra for various W-contained compounds
ꢁ1
2
3
2
3
3
2 3
O
conversion and products selectivity were calculated as the and supported W catalysts are presented in Fig. 1. Comparison
reaction reached steady state according to the following of the Na WO , AMT, and unsupported WO compounds with
2
4
3
equations:
the supported W catalysts could provide reliable information
about the local molecular coordination and bonding of sup-
ported tungsten oxide catalysts (Fig. 1a). In detail, Na WO
2 4
Conversion of 1-butene ð%Þ
À
Á
À
Á
À
Á
]
]
4
]
4
C3
þ 2 i ꢁ C
þ 2 2 ꢁ C
þ 4ðC5þ
Þ
n
n
n
n
possesses a single ligand-to-metal charge transfer (LMCT) band
¼
À
Á
À
Á
À
Á
À
Á
]
]
]
]
C3
þ 2 1 ꢁ C
4
þ 2 i ꢁ C
4
þ 2 2 ꢁ C
4
þ 4ðC5þÞn
maxima occurring at ꢂ208 nm; this represents the isolated
n
n
n
n
tetrahedral structure of WO . Ammonium metatungstate (AMT)
x
precursor indicates characteristic spectrum with two maxima at
Selectivity to propene ð%Þ
250 and 330 nm; this is related to the LMCT assigned to the
À
Á
]
C3
m
¼
À
Á
À
Á
À
Á
]
C3
]
4
]
4
þ i ꢁ C
þ 2 ꢁ C
þ ðC5þÞm
]
m
m
m
]
3
where (C
)
n
is the molar number of propene, (C
3
)
m
is the
weight percentage of propene in all hydrocarbon products. In
addition, the specic activity is also calculated as the converted
amount of 1-butene in molar base per gram of catalyst per hour.
3. Results and discussion
3.1 Effect of support character
Table S1† summarizes the BET specic surface area, pore
volume, pore size, W surface density, and acidity of as-prepared
supports and W-contained catalysts. It is clear that SBA-15
exhibits the highest specic surface area and pore volume of
2
ꢁ1
3
ꢁ1
7
19 m
g
and 1.09 cm g , respectively. In contrast, the
2
ꢁ1
corresponding values for TiO are the lowest at 13 m g and
.06 cm g . The structural properties of selected support
2
3
ꢁ1
0
ꢁ
2
directly determine the tungsten oxide surface density (W nm ).
As listed in Table S1,† the surface W density is linearly increased
with a decrease in the surface area, which is calculated by using
the initial BET surface area of the oxide support. For acidity,
the 8W/SBA-15 and 8W/SiO display the low content in Brønsted
acid which is not observed for 8W/g-Al and 8W/TiO . In spite
of absence of Brønsted acid, 8W/g-Al shows the signicantly
39
2
2
O
3
2
2 3
O
highest content in Lewis acid. The absence of Brønsted acid
over 8W/g-Al O catalyst is similar to the result reported by Xu
2
3
17
et al. In general, the content in Lewis acid is always much
higher than that for Brønsted acid over as-prepared W-
contained catalysts.
The XRD patterns of supported W catalysts are shown in
Fig. S1.† In the case of 8W/SiO
to monoclinic crystalline WO
which are located at 23.1, 23.6, 24.3, 28.9, 33.3, and 34.1 . Those
2
, the diffraction peaks assigned
3
(PDF#72–1465) appear clearly,
ꢀ
diffraction peaks could be observed but much weaker over 8W/
Fig. 1 UV-vis DRS spectra of (a) various W-containing compounds,
and (b) 8 wt% W-containing catalysts and their corresponding support
2
TiO catalyst (Fig. S1a and b†). However, both 8W/SBA-15 and
8W/g-Al O
2 3
catalysts do not show any diffraction peak for materials.
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oligomeric octahedral structure of WO species. For as-prepared
unsupported WO , it exhibits a broad band at ꢂ400 nm
3
41
attributed to d–d band of crystalline WO . As exhibited in
3
Fig. 1b, 8W/g-Al O gives rise to single band at ꢂ220 nm indi-
2
3
42
cating the presence of isolated tetrahedral WO
x
.
For W sup-
ported on TiO
for TiO
support except for a weak band at ꢂ400 nm which
suggests that a small amount of crystalline WO
2
, UV-vis spectra have no clear difference with that
2
43
3
is present. To
be signicantly different from above two catalysts, both 8W/
SiO and 8W/SBA-15 display a typical band at ꢂ225 nm with
2
a weaker band at ꢂ270 nm indicating the presence of the iso-
lated tetrahedral structure and the oligomeric octahedral
44
structure of WO
the 8W/SBA-15, 8W/SiO
crystalline WO
results.
x
species, respectively. However, compared to
exhibits one more band at ꢂ400 nm for
3
, which is well in agreement with the XRD
2
Fig. S2† illustrates the Raman spectra of various supports
and corresponding W-loaded catalysts compared at a similar
net intensity. It can be seen that 8W/TiO exhibits a weak new
2
ꢁ1
band for crystalline WO phase at ꢂ805 cm relative to that for
3
TiO
2 2 3
support. When W is loaded into g-Al O , a single band at
ꢁ1
ꢂ975 cm is assigned to the terminal n
s
(W]O) of surface
includes notable
45
WO
x
.
The Raman spectrum of 8W/SiO
2
ꢁ
1
bands at 274, 715, 805, and 980 cm ; these bands feature the
deformation vibration mode of W–O–W, bending vibration
mode of W–O, symmetric stretching vibration (n ) mode of W–O,
s
and symmetric stretching vibration mode of terminal W]O of
4
2
surface WO , respectively. 8W/SBA-15 exhibits signicantly
x
ꢁ1
weaker bands at 274, 715, and 805 cm compared to that for
ꢁ1
8
W/SiO
2
, and similar intensity of band at 980 cm . To this end,
over
we elucidated the structural differences of WO
x
supported W catalysts through the XRD patterns, UV-vis
spectra, and Raman spectra analysis.
The catalytic results of as-prepared W catalysts supported on
different supports for metathesis of ethylene and 1-butene are
shown in Fig. 2. The unsupported WO3 catalyst displays the
lowest 1-butene conversion and nearly no detectable selectivity
Fig.
2
Time-dependence of 1-butene conversion (a), propene
3
selectivity (b), and products distribution (c) obtained over WO and
to propene. In spite of substantially higher 1-butene conversion various 8 wt% W-containing catalysts at the reaction conditions of
ꢀ
ꢁ1
with a gradual deactivation over supported 8W/TiO
2
, there is 450 C, 0.1 MPa, 0.5 g of catalyst weight, 1.8 h of WHSV, and an
almost no propene formed via metathesis reaction. The 1-
butene is dominantly transformed into trans-2-butene and cis-2-
ethylene/1-butene molar ratio of 2.
butene via double bond isomerization over both WO and 8W/
3
TiO (Fig. 2c and Table S2†). According to the above results, it isomerization of 1-butene to iso-butene via skeletal isomeriza-
2
can be deduced that the crystalline WO did not provide active tion. The metathesis reaction is difficult to occur over iso-
3
46
sites for the metathesis reaction to convert isomerized 2-butene butene and ethylene. Besides, the presence of Brønsted acid
and ethylene into propene. Over 8W/TiO
catalyst, the catalytic sites might be favorable for promoting the metathesis reaction
results suggest that the metathesis reaction might be closely of 2-butene and ethylene for the formation of desired propene.
2
related to the terminal W]O of surface WO
x
, since the Raman Indeed, the higher selectivity to propene has been obtained over
2
and 8W/SBA-15 with the presence of suitable amount
present for 8W/TiO but clear for 8W/SiO , 8W/SBA-15, and 8W/ of Brønsted acid sites (Fig. 2c and Table S2†). As shown in Fig. 2,
at ꢂ980 cmꢁ is not 8W/SiO
1
band of it in isolated tetrahedral WO
x
2
2
g-Al O (Fig. S2b†), and the latter three catalysts show much 8W/SBA-15 indicates the highest 1-butene conversion and
2
3
higher selectivity to propene except for 8W/g-Al O . 8W/g-Al O
2 3
selectivity to propene, which can be explained from two aspects.
exhibits relatively lower metathesis activity compared to that for On the one hand, more Lewis and Brønsted acid sites are
W/SiO
and 8W/SBA-15, which possibly results from their available for 8W/SBA-15 compared to that for 8W/SiO ; on the
signicant difference in acidity. It can be seen that 8W/g-Al
other hand, the W highly dispersed on SBA-15 with large surface
contains appreciable number of Lewis acid sites with no area leads to more formation of the terminal W]O in isolated
detectable Brønsted acid sites (Table S1†), which favors the tetrahedral structure of surface WO (Fig. 1 and S2b†) due to
2
3
8
2
2
2 3
O
x
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more silanol group (Si–OH) interacted with the supported Fig. S6A† that all the samples exhibit the type IV isotherms with
3
0
surface WO species. Based on the abovementioned catalytic H1 hysteresis loops, which are typical for highly ordered
x
51
results and discussions, the metathesis reaction strongly hexagonal structure of mesoporous silica material. These
depends on the terminal W]O of isolated tetrahedral WO and results conrm the maintenance of ordered hexagonal
x
surface acidity.
arrangement of the SBA-15 frameworks upon W loading.
Fig. S6B† shows the pure SBA-15 and its supported W samples
have a narrow pore size distribution with an average size about
ꢂ6 nm. As the W loading increases from 4 to 30 wt%, the
surface density of W atoms is drastically enhanced from 0.18 to
3.2 Effect of calcination temperature
Fig. S3† shows the XRD patterns of 8W/SBA-15 prepared with
different calcination temperatures. It is clear that more crys-
talline WO phase is present on the surface of SBA-15 support
3
ꢁ
2
2.84 W nm . Regarding the strength and types of acid sites, it
can be determined from the pyridine adsorption measured by
IR spectra (Fig. S7†). As presented in Table 1, the acidity for both
Brønsted acid and Lewis acid rstly increases and then
decreases with a higher W loading and the 15 wt% W loading
leads to the maximum acidity. Note that the pure SBA-15 and
a low W loading of 4 wt% do not indicate the presence of
with increasing calcination temperature from 400 to 550, and
ꢀ
7
00 C as reected with the notable increase in diffraction peaks
ꢀ
at 23.1, 23.6, 24.3, and 34.1 ; this is ascribed to the sintering of
amorphous oligomeric octahedral WO species to form the
x
crystalline WO at higher temperature. Similarly, Praserthdam
3
et al. investigated the effects of calcination ramp rate and
calcination temperature on the amount of crystalline WO . It
was shown that a lower calcination ramp rate could enhance the
catalyst activity in metathesis reaction from ethylene and 2-
butene as compared to the ones calcined with a higher rate
Brønsted acid. As shown in Fig. S7,† pure SBA-15 shows a band
3
ꢁ1
at ꢂ1445 cm
assigned to hydrogen-bonded pyridine (H),
indicating that the H band seems to be come from the pyridine
hydrogen bonded to the Si–OH. With a loading of WO , a new
52
3
ꢁ1
band located at ꢂ1450 cm formed is attributed to pyridine
47
because they led to better dispersed catalysts. And the WO
crystals were also clearly detected in XRD at a high calcination
3
coordinately bonded to surface Lewis acid sites; these acid sites
are ascribed to the isolated WO
x
in the tetrahedral coordina-
is loaded in 8 wt%, a new band at
gradually appears, indicating the protonated
48
temperature. In our study, the speculated transformation of W
phase is well conrmed by their UV-vis DRS spectra as shown in
Fig. S4.† In detail, for 8W/SBA-15-700C catalyst, the relative
intensity of band at ꢂ270 nm to that at ꢂ225 nm is substantially
increased in contrast to that for 8W/SBA-15-400C and 8W/SBA-
53
tion. When the WO
3
ꢁ
1
ꢂ1540 cm
pyridine bonded to surface Brønsted acid. The appearing
surface Brønsted acid sites might be due to the protonation of
54
terminal W sites in the tetrahedral coordination. With
continuously increasing W loading, both Brønsted acid and
Lewis acid are decreased aer reaching the maximum acidity
1
5-550C catalysts. Also, a new pronounced band of crystalline
WO
at ꢂ400 nm for 8W/SBA-15-700C is detectable, which
means that the increased content in crystalline WO mainly
3
3
with a loading of 15 wt%; this is ascribed to the increased WO
content leading to the formation of oligomeric species and
further bulk WO species with lower content in the terminal W
3
comes from the evolution of oligomeric octahedral WOx
species. Raman spectra for those prepared 8W/SBA-15 catalysts
also demonstrate that the content of crystalline WO increases
with an increase in the calcination temperature (Fig. S5†).
It is somewhat curious that the abovementioned change of
the state of W phase has negligible effect on the both isomeri-
zation of 1-butene and subsequent metathesis reaction as pre-
sented in Table S3.† It is worthy of note that, prior to catalytic
3
3
sites. To this end, we have clearly elucidated the type and
content of acidity varied with the WO content.
x
In order to further conrm the evolution of W phase and its
molecular bonding with different W loadings, the as-prepared
supported W catalysts are characterized by XRD, Raman, and
XPS. The XRD patterns of SBA-15 and its supported W catalysts
are given in Fig. 3. It is clearly illustrated that the crystalline
reaction, the as-prepared 8W/SBA-15-400C is pretreated in
ꢀ
a ow of N at 550 C. The pretreatment procedure may lead to
2
WO
3 3
(PDF#83-0948, triclinic) is present when the WO content
no signicant discrepancies between 8W/SBA-15-400C and 8W/
is higher than 8 wt%, which is different from that (PDF# 72-
4
9
SBA-15-550C before the catalytic reaction. For 8W/SBA-15-
50C and 8W/SBA-15-700C catalysts, the similar catalytic
performance might be owing to their no distinct difference in
the content for the isolated tetrahedral WO (Fig. S4†). These
1
465, monoclinic) for 8W/SiO sample (Fig. S1†). The intensity
2
5
ꢀ
of diffraction peaks located at 2q of 23.1, 23.6, 24.3, and 33.6
becomes stronger with an increase in the WO
suggests that a certain amount of crystalline WO
presented in Fig. 4, the Raman spectra for above samples imply
that the aggregated WO species including amorphous and
3
content, which
appears. As
x
3
reasons are responsible for almost unchanged catalytic results
for 1-butene conversion and products selectivity.
x
crystalline W phase continuously increase with an increase in
3.3 Effect of W loading
the WO content, as reected with stronger bands at 274, 715,
3
ꢁ
1
ꢁ1
Table 1 shows the BET surface area, pore volume, pore size, and and 805 cm . The band at ꢂ980 cm assigned to terminal
acidity for the SBA-15 supported W catalyst with different W W]O bond rstly increases and then slightly decreases. The
loadings. The BET surface area and pore volume continuously strongest band is present over 15W/SBA-15 catalyst. This can be
decrease with an increase in the W loading due to the additional inferred that the concentration of highly dispersed WO
x
species
mass introduced by the supported WO phase. The reduced cannot be increased further, and the additional W loading
x
2
6
surface area possibly originates partially from bulky WO
x
contributes progressively to the growth of WO
3
nanoparticles.
50
species blocking the pores of SBA-15. It also can be seen from XPS measurements are conducted to detect the oxidation state
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Table 1 Physicochemical properties of SBA-15 and W/SBA-15 catalysts with different W loadings
ꢁ1
Acidity (mmol g
)
a
b
b
Surface area
Pore volume
Pore size
(nm)
Surface density
2
ꢁ1
3
ꢁ1
ꢁ2
c
c
Catalyst sample
SBA-15
(m g
)
(cm g
)
(W nm
)
Brønsted acid
Lewis acid
719
603
551
500
470
393
1.09
0.85
0.83
0.74
0.74
0.60
6.1
5.6
5.8
5.9
5.9
5.9
N/A
0.0
0.0
2.4
3.2
1.6
0.8
0.0
6.0
12.0
13.8
13.2
7.8
4
8
1
2
3
W/SBA-15
W/SBA-15
5W/SBA-15
0W/SBA-15
0W/SBA-15
0.18
0.41
0.92
1.39
2.84
a
b
c
Determined by BET method. Evaluated by the BJH method. Determined by pyridine-IR measurement.
ascribed to the 4f7/2 and 4f5/2 peaks of W atoms for aggregated
6
+
6+
W
oxide and dispersed W hydroxide WO
3
(OH
2 n
) , respec-
56,57
6+
tively.
transformed to surface tetrahedral WO
catalysts of 4W/SBA-15, 15W/SBA-15, and 30W/SBA-15 exhibit
the peaks of W hydroxide, its peaks intensity displays maxima
over 15W/SBA-15 catalyst, which can be explained that the iso-
lated tetrahedral WO is active to absorb H O molecules in air
As illustrated in literature, the W hydroxide could be
58
x
.
The investigated
6
+
x
2
for the formation of WO
3
(OH
2
)
n
. This result further conrms
x
that 15W/SBA-15 has the highest content of isolated WO .
The catalytic performance over SBA-15 supported W catalysts
with different loadings is summarized in Fig. 6 and Table S4.†
The pure SBA-15 exhibits a stable 1-butene conversion of 72.3%
with a negligible selectivity to propene. The 1-butene is mainly
converted into trans-2-butene and cis-2-butene via C]C bond
isomerization, which suggests that the Si–OH in SBA-15 has the
52
isomerization activity. As W is introduced to SBA-15 with
Fig. 3 XRD patterns of fresh SBA-15 and W/SBA-15 catalysts with a 4 wt% loading, the selectivity to propene is notably enhanced
different W loadings.
from 2.9 to 37.4%, however, the 1-butene conversion increases
slightly from 72.3 to 78.9% (Table S4†). This result reects that
the isolated WO of tetrahedral coordination is responsible for
x
the metathesis reaction to produce propene. With further
Fig. 4 Raman spectra of fresh SBA-15 and W/SBA-15 catalysts with
different W loadings.
of surface W species. The XPS spectra of W 4f level are tted
55
according to the theory of Doniach and Sunjic. As shown in
Fig. 5 XPS spectra of fresh 4W/SBA-15, 15W/SBA-15, and 30W/SBA-
Fig. 5, the binding energy at 35.7/37.9 eV and 36.2/38.1 eV is 15 catalysts.
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catalysts are characterized by UV-vis DRS spectra as presented
in Fig. S8.† It is clear that the intensity of band at ꢂ225 nm
gradually increases to a stable level when W loading is below
15 wt%, which is attributed to the increased content for the
isolated tetrahedral WO structure during the catalytic reaction.
x
In addition, all spent catalysts exhibit gradually increased
intensity of the band in the range of 400 to 700 nm. The
higher W content supported on SBA-15, the more markedly
4
+
elevated band intensity. This band is assigned to the W and
5
+
17
W
species. The lower oxidation state of W species is due to
6
+
the reduction of W during the reaction. To further conrm the
presence of lower oxidation state of W species, the spent 15W/
SBA-15 catalysts with different reaction time are measured by
XRD as displayed in Fig. 7. One can see that the diffraction peak
ꢀ
at 2q of 33.6 assigned to crystalline WO
3
is gradually shied to
ꢀ
lower value at 33.3 due to the appearance of nonstoichiometric
phase WO2.92 and WO2.83, which suggests that the tungsten
oxide species are gradually reduced during the metathesis
reaction. It can be inferred that the reduction of W catalysts
59
might be closely related to the induction period. As shown in
Fig. S9,† the TPR proles of fresh 8W/SBA-15, 15W/SBA-15, and
3
0W/SBA-15 catalysts display three typical reduction peaks for
60
3 2 2
WO to WO2.90, WO2.90 to WO , and WO to W, respectively.
The temperature range for three different reduction stages is
ꢀ
located at 400–500, 630–740, and 740–880 C, respectively. This
result further demonstrates that the tungsten oxide species
ꢀ
could be reduced at the reaction temperature of 450 C during
metathesis reaction.
3.4 Effect of pretreatment gas
The catalytic performance for 15W/SBA-15 pretreated with
different gases is presented in Fig. 8 and Table 2. In contrast to
N -pretreated 15W/SBA-15 previously described as a reference,
2
H -pretreatment does not signicantly affect the 1-butene
2
conversion, however, the induction period becomes shorter to
Fig.
6 Time-dependence of 1-butene conversion (a), propene
selectivity (b), and products distribution (c) obtained over SBA-15 and
W/SBA-15 catalysts with different W loadings at the reaction condi-
ꢀ
ꢁ1
tions of 450 C, 0.1 MPa, 0.5 g of catalyst weight, 1.8 h of WHSV, and
an ethylene/1-butene molar ratio of 2.
increase in W loading from 4, 8 to 15 wt%, the 1-butene
conversion continuously increases to a small extent from 78.9,
83.3, to 88.2%. Accordingly, the selectivity to propene increases
remarkably from 37.4, 54.6, to 68.6% with marked increase in
Brønsted acidity (Table 1). The above discussion demonstrates
that both Lewis and Brønsted acid sites from the isolated
tetrahedral WOx structure are favorable for the metathesis
reaction. When W is introduced over a suitable amount, for
example 15 wt%, both 1-butene conversion and selectivity to
propene are decreased. In contrast, the metathesis activity is
reduced to a larger extent. This is ascribed to the less amount of
the formation of isolated tetrahedral WO
results from its evolution into the inactive crystalline WO
In order to investigate the evolution of SBA-15 supported W
x
structure which
3
.
Fig. 7 XRD patterns of fresh 15W/SBA-15 and two spent catalysts
catalysts during the catalytic reaction, the fresh and spent recovered after 40 min and 10 h of reaction, respectively.
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result in a higher initial selectivity to propene at 30% compared
with that for N -pretreatment at 20%. For 15W/SBA-15 pre-
2
treated by 1% O /N gas, the 1-butene conversion is still kept
2
2
unchanged. This indicates that the 1-butene isomerization is
not closely related to the state of WO . As expected, the initial
x
selectivity to propene obtained in metathesis reaction is
signicantly lower at 5% and need a longer time to reach the
stable stage. The abovementioned result analysis strongly
x
demonstrates that the partially reduced WO species is neces-
sary precursor for the formation of active sites for metathesis
reaction, but not for 1-butene isomerization. Liu et al. also
conrmed that the partially reduced WO2.92 as active phase for
2
metathesis reaction by adjusting the H concentration in prior
61
reduction process. It is no doubt that the extent of reduction
inuences the metathesis activity as previously mentioned.
However, it is hard to correlate the metathesis activity with
x
quantifying the content of reduced WO species. Indeed,
without prior reduction process, the active sites for metathesis
reaction are generated by the interaction of olens with W/SBA-
15 catalyst during the reaction, which takes more time for
subsequent formation of propene.
Interestingly, the pretreatment with water vapor introduced
2
by a owing N leads to the complete deactivation of metathesis
reaction (Fig. 8b), however, the isomerization activity is similar
to that for pure SBA-15 support (Fig. 6a). Furthermore, the spent
ꢀ
H O-pretreated 15W/SBA-15 catalyst is calcined in air at 550 C
2
like the preparation procedure for fresh catalyst. The obtained
sample is re-evaluated using the same reaction conditions with
the fresh N
the catalytic performance for 1-butene conversion and selec-
tivity to propene. This result indicates that H O-pretreatment
seriously alters the existing structure of WO species (Fig. 9a).
2
For comparison, the XRD patterns for spent N -, H -, and O -
2
-pretreated catalyst, which has not fully recovered
2
x
2
2
pretreated 15W/SBA-15 catalysts do not exhibit obvious evolu-
tion for tungsten oxide species (Fig. 9b). On the other hand, the
diffraction peaks for H O-pretreated 15W/SBA-15 catalyst illus-
2
trate the presence of large amount of crystalline WO
These intensive peaks are located at 23.1, 23.6, 24.3, and 33.3 ,
3
phase.
Fig.
8
Time-dependence of 1-butene conversion (a), propene
ꢀ
selectivity (b), and products distribution (c) over 15W/SBA-15 catalysts
pretreated in different gases at the reaction conditions of 450 C,
ꢀ
which represent the monoclinic WO
3
similar to that for 8W/SiO
O-pretreated
2
ꢁ
1
(Fig. S1†). XPS spectra of fresh 15W/SBA-15 and H
2
0
.1 MPa, 0.5 g of catalyst weight, 1.8 h of WHSV, and an ethylene/1-
a
butene molar ratio of 2. Spent 15W/SBA-15 catalyst pretreated in H
2
O
15W/SBA-15 catalysts are presented in Fig. S10.† In the case of
followed by calcination, and the obtained catalyst further treated in N
stream before reaction.
2
H O-pretreated 15W/SBA-15 catalyst, the peaks intensity of
binding energy at 35.7 and 37.9 eV (black line) substantially
2
ꢀ
Table 2 Catalytic performance of 15W/SBA-15 catalyst pretreated in different gases at 550 C
Selectivity (%)
Pretreatment
gas
Specic activity
]
a
Conversion (%)
Propene
C
5+
trans-2-Butene
cis-2-Butene
Iso-butene
4
(mmol C per gcat per h)
N
H
O
H
H
2
88.2
85.9
87.7
72.4
76.5
68.6
62.0
67.1
0.9
4.5
4.2
4.6
0.5
3.0
15.2
16.5
16.1
56.2
35.0
11.1
12.1
11.7
41.4
26.4
0.5
1.5
0.7
1.1
0.4
14.2
13.8
14.1
11.6
12.3
2
2
2
O
O
b
2
35.2
a
ꢀ
ꢁ1
b
Reaction conditions: T ¼ 450 C, P ¼ 0.1 MPa, catalyst weight ¼ 0.5 g, WHSV (E + B) ¼ 1.8 h , n(E)/n(B) ¼ 2. Spent 15W/SBA-15 catalyst
2 2
pretreated in H O followed by calcination, and the obtained catalyst further treated in N stream before reaction.
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irreversible deactivation due to the sintering of WO
x
species; on
the other hand, the remaining presence of H O may also poison
2
5
+
the reduced W sites to be interacted with olens for further W-
carbene formation. Although the catalyst was further ushed
ꢀ
with inert N
2
at relatively high temperature of 450 C before the
reaction, the metathesis reaction cannot proceed.
3.5 Effect of WHSV of ethylene and 1-butene
In order to obtain the optimal catalytic performance for the
metathesis reaction of ethylene and 1-butene over 15W/SBA-15
catalyst, we have investigated the inuence of WHSV of mixed
feed gas in a xed-bed reactor. The catalytic performances are
evaluated by 1-butene conversion, specic activity and product
selectivity. Note that the specic activity is dened as the
converted amount of 1-butene in molar base per gram of
catalyst per hour. In our study, the WHSV was regulated by
changing the loaded amount of catalyst with a constant ow
ꢁ
1
rate of feed gas at 9 mL min (molar ratio ethylene/1-butene
2/1). As presented in Fig. S12† and Table 3, the 0.25 g of
¼
ꢁ
1
1
5W/SBA-15 (WHSV ¼ 3.6 h ) leads to the lowest 1-butene
conversion and remarkably lower selectivity to propene but the
highest specic activity at 25.7 mmol C4 per gcat per h. The 1-
]
butene conversion and selectivity to propene reach the
ꢁ
1
maximum value at 91.2% and 75.8% with a WHSV of 0.9 h
,
respectively. Accordingly, the specic activity is about
]
7
6
.4 mmol C
.4 mmol C
4
]
per gcat per h, which is higher than the value of
4
per gcat per h for W-FDU-12 combined with MgO
3
3
catalysts under the similar reaction conditions. However, the
ꢁ
1
overloaded catalyst (1.5 g, WHSV ¼ 0.6 h ) slightly decreases
both 1-butene conversion and selectivity to propene with the
]
lowest specic activity at 4.8 mmol C
4
per gcat per h. On the
Fig. 9 XRD patterns of (a) fresh 15W/SBA-15, H
SBA-15, spent H O-pretreated 15W/SBA-15, calcined spent H
2 2
2
O-pretreated 15W/
O-
pretreated 15W/SBA-15, and (b) spent 15W/SBA-15 catalysts pretreated
in different gases (N , H , O and H O).
basis of above catalytic results, the insufficient loaded catalyst
exhibits much lower selectivity to propene, which suggests
that the reaction rate of metathesis step is slower than that for
isomerization step. In other words, the isomerized 2-butene
could not be quickly converted into propene via metathesis
reaction. This conclusion can also be obtained from the
changing trend of 1-butene conversion, propene selectivity,
and specic activity as listed in Table 3. As the WHSV is raised
2
2
2
2
6
+
increases, which suggests that the content for aggregated W
oxide increases. Correspondingly, the peaks intensity of
binding energy at 36.2 and 38.1 eV (blue line) decrease to
6
+
ꢁ1
a remarkable extent with the decreased content for W
hydroxide WO (OH . Despite that WO (OH still exists, the
O-pretreated 15W/SBA-15 catalyst does not display metath-
esis activity. This is ascribed to the remaining presence of H
molecules at the surface of catalyst to be unfavorable for the
transformation of WO (OH into active sites for metathesis
reaction. In order to clarify the inuence of H O on the catalyst
to a much higher value at 3.6 h , the 1-butene conversion
decreases slightly while the selectivity to propene drops
remarkably from 72.7 to 40.6%. Seen from the specic activity
3
2
)
n
3
2 n
)
H
2
ꢁ
1
2
O
listed, the high WHSV of 3.6 h exhibits about 5 times higher
ꢁ
1
value than that for 0.6 h . This result also indicates that the
conversion rate of 1-butene is quite fast. The metathesis
reaction is rate-determining step for the enhancing selectivity
to propene. In the case of overloaded catalyst, the selectivity to
propene is decreased due to the reversible conversion between
3
2 n
)
2
evolution, the H -TPR experiments for fresh 15W/SBA-15 and
2
calcined spent H O-pretreated 15W/SBA-15 catalysts are carried
2
3
5
out. As presented in Fig. S11,† the rst reduction peak at the
propene, 2-butene, and 1-butene. As the formed propene
could not be run away from the catalyst surface, it will return
back to 2-butene and ethylene. It is worth noting that a longer
residence time (lower WHSV) gradually enhances the skeletal
isomerization of 1-butene, despite that it is more difficult to
ꢀ
range of 400–500 C shis to higher temperature range and the
peak intensity becomes much weaker. This result shows that
the H O pretreatment seriously affects the reducibility of
2
tungsten oxide species. As previously demonstrated, the
reduction behavior does really play an important role in form-
ing the active sites for metathesis reaction. In our viewpoint, on
4
0
proceed than that for C]C bond isomerization. Therefore,
a suitable WHSV is necessary to obtain the high selectivity to
propene.
the one hand, the catalyst pretreated by H O leads to the
2
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Table 3 Catalytic performance of 15W/SBA-15 catalyst at different WHSV
Selectivity (%)
Specic activity
(mmol C per gcat per h)
4
ꢁ1
a
]
WHSV (h
)
Conversion (%)
Propene
C
5+
trans-2-Butene
cis-2-Butene
Iso-butene
0
0
1
3
.6
.9
.8
.6
90.6
91.2
88.2
79.8
72.7
75.8
68.6
40.6
4.3
4.1
4.5
4.2
12.3
10.9
15.2
31.5
8.9
7.8
11.1
23.4
1.7
1.2
0.5
0.3
4.8
7.3
14.2
25.7
a
ꢀ
Reaction conditions: T ¼ 450 C, P ¼ 0.1 MPa, catalyst weight ¼ 1.5, 1.0, 0.5, and 0.25 g, n(E)/n(B) ¼ 2.
3
.6 Effect of reaction temperature
isomerization activity, which is catalyzed by Si–OH (Scheme 1,
). The Si–OH might be working as a weak Brønsted acid site for
isomerization of C]C bond, however, lack of ability of
metathesis. Table S4† shows that the varied W loadings do not
affect the level of 1-butene conversion in spite of the less
amount of Si–OH on W/SBA-15 with well dispersed W over the
support. As noted in the previous description, the isomerization
rate of 1-butene is so fast that the decrease in the amount of Si–
OH could not suppress the 1-butene isomerization. It is
1
Finally, the reaction temperature is regulated to optimize the
catalytic performance over 15W/SBA-15 catalyst under the above
63
ꢁ
1
optimal WHSV of 0.9 h . As can be seen from Fig. S13† and
Table 4, the lower reaction temperature gives slightly lower 1-
butene conversion but signicantly lower selectivity to propene.
The decreased trend in 1-butene conversion is much milder
than that for propene selectivity, which illustrates that the
metathesis reaction is more difficult to occur relative to the
isomerization reaction of 1-butene. The optimal selectivity to
propene at 79.1% with 1-butene conversion of 92.1% is ob-
tained over the reaction temperature of 500 C. When the
reaction temperature is further elevated to 550 C, in spite of the
stable 1-butene conversion, the selectivity to propene is obvi-
ously decreased due to the formation of C –C alkane from
1 3
thermal-cracking. Thereby, the suitable reaction temperature
should be selected to obtain the highest selectivity to propene
through efficient conversion of the formed intermediate 2-
butene into propene via metathesis reaction, and avoid the side-
reactions such as skeletal isomerization and thermal-cracking.
3
important to note that unsupported WO displays the negligible
activity for 1-butene isomerization and inactive in metathesis
reaction. The previously mentioned results suggest that the
ꢀ
6
+
ꢀ
isolated tetrahedral WO
x
(W ) species can be reduced by olen
5
+
or H
2
to form the W species, which might be regarded as the
pre-requisite step for subsequent formation of W-carbene
(Scheme 1, 2). Indeed, this viewpoint is strongly supported by
following experimental facts: (1) shorter induction period for
62
formation of propene aer the catalyst pretreated in H
2
stream;
(
2) pretreatment with O results in lower initial selectivity to
2
5
+
propene (Fig. 8); (3) observed presence of W in spent catalysts
from UV-vis DRS spectra and XRD patterns (Fig. 7 and S8†)
6
+
compared to fresh catalyst with only W species (Fig. 5). It
should be noted that the pretreatment with H O directly deac-
Based on above experimental results combined with various tivates the metathesis activity completely. The deactivation of
characterizations such as XRD, UV-vis DRS, Raman, pyridine IR, catalyst might mainly result from the H O-induced aggregation
and H
-TPR, it is well conrmed that the partially reduced of isolated WO species as the strong diffraction peaks for
isolated tetrahedral WO
species can be responsible for subse- inactive WO phase are observed before and aer reaction
quent metathesis reaction of 1-butene and ethylene.
(Fig. 9a). However, the metathesis activity could be recovered to
When the 1-butene and ethylene are used as reactants for some extent due to the resulting re-dispersion of aggregated
3.7 Discussion on mechanism of metathesis reaction
2
2
2
x
x
3
58
producing propene via metathesis reaction, there are three WO
steps involved including (1) fast isomerization of 1-butene, (2) the calcined spent H
carbene formation, and (3) metathesis reaction as shown in weaker diffraction peaks for crystalline WO
3
phase by calcination treatment. As reected in Fig. 9a,
O-pretreated 15W/SBA-15 exhibits much
phase compared to
2
3
Scheme 1. It is no doubt that SBA-15 exhibits signicant that before calcination treatment. In spite of the proposed route
Table 4 Catalytic performance of 15W/SBA-15 catalyst at different reaction temperatures
Selectivity (%)
Specic activity
(mmol C per gcat per h)
4
ꢀ
a
]
T ( C)
Conversion (%)
Propene
C
5+
trans-2-Butene
cis-2-Butene
Iso-butene
3
4
4
5
5
50
00
50
00
50
79.6
84.6
91.2
92.1
92.3
12.5
49.5
75.8
79.1
72.8
1.8
4.2
4.1
3.8
5.9
49.6
26.0
10.9
8.7
34.3
18.6
7.8
6.5
6.0
1.8
1.7
1.2
1.9
3.1
6.4
6.8
7.3
7.4
7.4
7.8
a
Reaction conditions: T ¼ 350–550 C, P ¼ 0.1 MPa, catalyst weight ¼ 1.0 g, WHSV (E + B) ¼ 0.9 hꢁ1, n(E)/n(B) ¼ 2.
ꢀ
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Scheme 1 Proposed route for conversion of 1-butene and ethylene into propene via isomerization (1), carbene formation (2), and metathesis
reaction (3) over SBA-15 supported W catalysts.
for formation of W-carbene, we cannot deny that the isolated metallocyclobutane intermediates by [2+2] cycloaddition reac-
6
+
5+
68,69
tetrahedral W species possibly play a similar role like W
species.
tion.
Subsequently, these intermediates can be decomposed
into propene with generation of new W-carbene, such as W]
5
+
The abovementioned isolated tetrahedral W species consist CH and W]CH–CH CH . The newly generated W]CH and
2
2
3
2
of two types of structure, such as the terminal W]O (Lewis acid W]CH–CH CH further provide the active sites for cycloaddi-
2
3
sites) and W–OH (Brønsted acid sites). It should be noted that tion with 2-butene to result in the metallocyclobutane inter-
these acid sites are possible to catalyze C]C bond isomeriza- mediates. Similarly, the propene and 2-pentene are liberated
64
tion. The widely accepted olen metathesis mechanism from the metallocyclobutane intermediates with re-generation
involves metallocarbene intermediates, implying that the of initial W]CH–CH active sites. To this end, the metathesis
3
successful activation of supported-oxide based olen metathesis reaction process for conversion of 1-butene and ethylene into
catalysts requires the conversion of W]O and W–OH species propene has been fully elucidated in this study.
65,66
into W-carbene.
The formation pathways of W-carbene over
W/SBA-15 catalysts are illustrated in Scheme S1.† According to
4
. Conclusions
the DFT calculation, the 2-butene preferentially forms W-carbene
67
active sites. In the case of terminal W]O, the W]O is rstly
combined with 2-butene by [2+2] cycloaddition reaction to form
a four-membered oxametallacycle intermediate. Subsequently,
the decomposition of the four-membered oxametallacycle
The investigation of the structure of tungsten oxide supported
on SBA-15 and its catalytic performance in the metathesis
reaction of 1-butene and ethylene to propene has been carried
out extensively. The main conclusions are summarized as
following several aspects.
For converting 1-butene and ethylene into propene, the 1-
butene is rstly required to be isomerized to 2-butene. The
results show the Si–OH serving as the primary active centers for
intermediate produces the acetaldehyde (H C–CH]O) and the
3

nal W]CH–CH active sites (Scheme S1A†). For the terminal
3
W–OH, the 2-butene is reacted with W–OH to form 2-butanoxide
by protonation reaction. Then, b-H in 2-butanoxide is combined
with W]O to form the six-membered oxametallacycle interme-
diate which can decompose into the four-membered oxame-
tallacycle intermediate and W–OH via migration of hydrogen
from 2-butene to W]O. Also, subsequent elimination of
1
-butene isomerization. And the terminal W]O (Lewis acid
sites) and W–OH (Brønsted acid sites) in the isolated tetrahedral
WO species might also play a role in the isomerization reac-
tion. Over investigated W/SBA-15 catalysts, the isolated tetra-
x
6
+
carbonyl compound (H C–CH]O) from the intermediate occurs
3
x 2
hedral WO (W ) species are partially reduced by olens or H
to give rise to WO (W ) species as the precursor sites for the
5
+
to form W]CH–CH active sites (Scheme S1B†). The above W-
3
x
carbene formed via both the terminal W]O and W–OH is
regarded as the active sites for metathesis reaction.
formation of W-carbene species which does contribute to olen
metathesis activity. In comparison to the isolated WO species,
x
The postulated metathesis mechanism for the formation of
propene is presented in Scheme S2.† It is clear that the initially
the crystalline WO phase does not show the metathesis activity
3
despite that they are partially reduced during the reaction.
The W/SBA-15 catalysts exhibit different initial selectivity to
propene with tuning pretreatment atmosphere. The reducing
formed W]CH–CH
3
active sites can react with ethylene
(Scheme S2a†) and 1-butene (Scheme S2b†) to give the
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gas H
2
results in a shorter induction time compared to that for 12 F. Jiang, B. Liu, W. P. Li, M. Zhang, Z. J. Li and X. H. Liu,
inert gas N2 and oxidizing gas O , which provides strong
Catal. Sci. Technol., 2017, 7, 4609–4621.
evidence that the partially reduced isolated WO (W ) species is 13 J. Zheng, J. Cai, F. Jiang, Y. B. Xu and X. H. Liu, Catal. Sci.
2
5
+
x
crucial for the subsequent generation of W-carbene. It is further
conrmed by H O pretreatment as the suppressed reduction of 14 W. G. Zhou, J. Y. Liu, X. Wu, J. F. Chen and Y. Zhang, Catal.
WO species results in complete loss of metathesis activity. The
sintering of isolated WO
in the crystalline WO
Technol., 2017, 7, 4736–4755.
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Commun., 2015, 60, 76–81.
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3
content, which is also possibly respon- 16 E. Mazoyer, K. C. Szeto, N. Merle, S. Norsic, O. Boyron,
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methods and reaction conditions, the 1-butene conversion of
J. M. Basset, M. Taouk and C. P. Nicholas, J. Catal., 2013,
301, 1–7.
9
1
7
2.1% and propene selectivity of 79.1% have been achieved over 17 S. J. Huang, S. L. Liu, W. J. Xin, J. Bai, S. J. Xie, Q. X. Wang
5W/SBA-15 catalyst. Accordingly, the specic activity is about
and L. Y. Xu, J. Mol. Catal. A: Chem., 2005, 226, 61–68.
2014, 50, 9060–9063.
]
.4 mmol C
4
per gcat per h. The present study provides the 18 T. Hahn, E. V. Kondratenko and D. Linke, Chem. Commun.,
molecular level structure–activity/selectivity relationship for the
1
-butene/ethylene metathesis to propene. The comprehensive 19 X. J. Li, W. P. Zhang, S. L. Liu, S. J. Xie, X. X. Zhu, X. H. Bao
insights will guide us to develop advanced W-based catalysts for and L. Y. Xu, J. Mol. Catal. A: Chem., 2009, 313, 38–43.
olen metathesis to remarkably enhance the selectivity to 20 C. Meetings, Chemistry, 2003, 9, 971–975.
desired olens.
21 S. Lwin and I. E. Wachs, ACS Catal., 2015, 6, 272–278.
2
2
2 S. Lwin and I. E. Wachs, ACS Catal., 2014, 4, 2505–2520.
3 S. J. Huang, F. Chen, S. L. Liu, Q. Zhu, X. Zhu, W. J. Xin,
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Conflicts of interest
There are no conicts to declare.
2
4 J. F. Wu, A. Ramanathan, W. K. Snavely, H. D. Zhu, A. Rokicki
and B. Subramaniam, Appl. Catal., A, 2016, 528, 142–149.
Acknowledgements
25 V. Goelden, D. Linke and E. V. Kondratenko, ACS Catal.,
015, 5, 7437–7445.
6 S. Lwin, Y. Li, A. I. Frenkel and I. E. Wachs, ACS Catal., 2016,
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2
The authors wish to acknowledge the nancial support from the
National Natural Science Foundation of China (21576119), the
Natural Science Foundation of Jiangsu Province (BK20151125),
the Fundamental Research Funds for the Central Universities
2
6
2
2
7 S. Lwin and I. E. Wachs, ACS Catal., 2016, 7, 573–580.
8 D. R. Hua, S. L. Chen, G. M. Yuan, Y. L. Wang, Q. F. Zhao,
X. L. Wang and B. Fu, Microporous Mesoporous Mater.,
(
(
JUSRP51720B), the Jiangsu Provincial Six Talent Peaks Program
XNY-001), and the Programme of Introducing Talents of
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Discipline to Universities (111 Project B13025).
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