Remarkably enhanced performance of the metathesis reaction of ethylene and 1-butene to propene using one-step prepared W-MCM-41 catalysts

Article abstract of DOI:10.1039/c9ra08646c

Highly dispersed tungsten species with an isolated tetrahedral WO_x species structure are substantially beneficial for the metathesis reaction of ethylene and 1-butene to propene. The conventional impregnation method always leads to the formation of inactive crystalline WO₃ thereby notably decreasing the amount of active sites. In this study, we synthesized a highly dispersed W-MCM-41 catalyst using the one-step precipitation method with a Si/W ratio of 30. The prepared catalyst showed excellent catalytic performance with a 1-butene conversion of 92.7% and a propene selectivity of 80.8%. In contrast, the impregnated catalyst with the same W loading as the one-step precipitation method resulted in a much lower 1-butene conversion of 76.5% and propene selectivity of 34.1%. Various characterization techniques including XRD, XPS, ICP-OES, UV-vis DRS, TEM, and Raman spectroscopy were applied to confirm that the one-step precipitation method can efficiently prepare well-dispersed W-MCM-41 catalysts with the desired structure in spite of the fact that the ideal dispersive structure was strongly dependent of the Si/W ratio and stirring time of the reaction mixture of tungstic acid and TEOS. In addition, the introduction of an upstream catalyst onto the W-MCM-41 catalyst could not obviously improve the 1-butene conversion and propene selectivity, which might be due to fast 1-butene isomerization easily occurring on the abundant Si-OH of the W-MCM-41 catalyst. This work provides new insights for the design of metathesis catalysts and reaction processes to efficiently convert ethylene and 1-butene into propene.

Full text of DOI:10.1039/c9ra08646c

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PAPER
Remarkably enhanced performance of the
metathesis reaction of ethylene and 1-butene to
propene using one-step prepared W-MCM-41
catalysts
Cite this: RSC Adv., 2019, 9, 40618
a
b
b
b
Hanxu Li*a
Changji Wang,† Feng Jiang,† Guangzheng Zuo,† Bing Liu,
b
and Xiaohao Liu
*
Highly dispersed tungsten species with an isolated tetrahedral WO
beneficial for the metathesis reaction of ethylene and 1-butene to propene. The conventional
impregnation method always leads to the formation of inactive crystalline WO thereby notably
x
species structure are substantially
3
decreasing the amount of active sites. In this study, we synthesized a highly dispersed W-MCM-41
catalyst using the one-step precipitation method with a Si/W ratio of 30. The prepared catalyst showed
excellent catalytic performance with a 1-butene conversion of 92.7% and a propene selectivity of 80.8%.
In contrast, the impregnated catalyst with the same W loading as the one-step precipitation method
resulted in a much lower 1-butene conversion of 76.5% and propene selectivity of 34.1%. Various
characterization techniques including XRD, XPS, ICP-OES, UV-vis DRS, TEM, and Raman spectroscopy
were applied to confirm that the one-step precipitation method can efficiently prepare well-dispersed
W-MCM-41 catalysts with the desired structure in spite of the fact that the ideal dispersive structure was
strongly dependent of the Si/W ratio and stirring time of the reaction mixture of tungstic acid and TEOS.
In addition, the introduction of an upstream catalyst onto the W-MCM-41 catalyst could not obviously
improve the 1-butene conversion and propene selectivity, which might be due to fast 1-butene
isomerization easily occurring on the abundant Si–OH of the W-MCM-41 catalyst. This work provides
new insights for the design of metathesis catalysts and reaction processes to efficiently convert ethylene
and 1-butene into propene.
Received 22nd October 2019
Accepted 3rd December 2019
DOI: 10.1039/c9ra08646c
rsc.li/rsc-advances
a highly versatile process for interconverting olenic hydro-
1
. Introduction
1
2
carbons. Propene can be formed through the metathesis
1
3–15
Propene is one of the most important building blocks in the reaction of ethylene and 1-butene.
Therefore, the metath-
chemical industry. The demand for propene has been esis reaction of ethylene and 1-butene to propene is a prom-
1
–7
continuously increasing at an average rate of 4–5% per year.
The Fischer–Tropsch to olens (FTO) process to produce light FTS products.
olens has attracted considerable interest because of the
Supported WO
availability of coal-, natural gas-, and biomass-derived syngas stability, anti-poisoning property, and low price and thus are
ising method to increase the selectivity toward propene from
catalysts exhibit better activity, excellent
3
8
,9
16–21
(
CO + H
2
).
However, the FTO route usually generates highly desirable for industrial application.
species and its existing
Despite the high selectivity to total light state on supports play critical roles in the catalytic perfor-
It is widely
different kinds of light olens, including ethylene, propene, accepted that the highly dispersed WO
x
1
0,11
and 1-butene.
22
olens, the selectivity to propene is always lower than 40%. mance. The properties of supports and the preparation
Therefore, it is necessary to develop an efficient approach to methods have a signicant effect on the structure and disper-
enhance the propene fraction and simultaneously suppress sion of WO
the generation of ethylene or 1-butene. Olen metathesis is lytic performance. Mesoporous materials, such as HMS, SBA-
5, MCM-41, and FDU-12, have uniform pores, large BET
species, and thus signicantly inuence the cata-
x
23
1
surface areas and pore volumes, which are benecial for the
a
dispersion of active species and the transportation of reactants
College of Earth and Environment, Anhui University of Science and Technology,
24–29
2
b
32001 Huainan, China
and products.
Thus, mesoporous materials have been
Department of Chemical Engineering, School of Chemical and Material Engineering, considered as promising catalyst supports in olens
Jiangnan University, Wuxi 214122, China. E-mail: liuxh@jiangnan.edu.cn
metathesis.
†
These authors contributed equally to this work.
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ꢀ
In our previous study, we investigated the structure of solid was calcined in air at 650 C for 4 h with a heating rate of
ꢀ
ꢁ1
tungsten oxide supported on SBA-15 and its catalytic perfor- 1 C min to obtain W-MCM-41(50) catalyst powder.
mance in the metathesis reaction of 1-butene and ethylene to As a comparison of the one-step prepared W-MCM-41(30)
30
propene. It was found that the propene production through catalyst, the impregnated 18W/MCM-41 is also prepared with
the metathesis reaction consists of three steps, including 1- the same loading of W in the two catalysts. The catalyst support,
butene isomerization, W–carbene formation, and metathesis MCM-41, is prepared as the above W-MCM-41(z) catalyst without
reaction. The silanol group (Si–OH) in SBA-15 is acting as the the addition of W precursor. The W species was impregnated into
active site for 1-butene isomerization. The W–carbene (W]CH– the MCM-41 support with the W loading of 18 wt% by an incip-
30
3
CH ) species are formed via the partial reduction of isolated ient wetness impregnation method as our previous work.
tetrahedral WO species which contain W]O or W–OH bonds.
x
The W–carbene species play a crucial role in the metathesis
2.2 Catalyst characterization
reaction. Furthermore, the W/SBA-15 pretreated by H O leads to
a decrease of the metathesis activity. This is mainly attributed to
2
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
the sintering of isolated WO
x
species, forming an inactive
crystalline WO phase. These results suggest that the nature of
3
the supported tungsten oxide phase signicantly affects the
catalytic performance of the olen metathesis reaction. Estab-
lishing the structure–activity/selectivity relationships for
metathesis reaction would be helpful for the rational design of
more efficient catalysts.
In present investigation, MCM-41 molecular sieve with two-
dimensional pore structure, smaller pore size and larger
surface area was used as the catalyst support to study the olen
ꢀ
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
(UV-vis DRS) were obtained in the range of 200–700 nm with
a Shimadzu UV-3600 Plus Spectrometer, using BaSO as refer-
4
ence. The Raman spectra of the samples were recorded on
a Renishaw Micro Raman Spectrometer in the range from 200 to
ꢁ
1
1
200 cm (excitation line: 785 nm of diode solid-state laser).
Transmission electron microscopy (TEM) images of the cata-
lysts were taken in a Tecnai G2 F30 S-Twin TEM operating at 200
kV. The XPS data were collected using X-ray photoelectron
spectrometer Thermo SCIENTIFIC ESCALAB 250xi with an Al-
Ka (1486.8 eV) X-ray source. ICP-OES were performed on an
Agilent 5100 inductively coupled plasma-optical emission
spectrometer.
x
metathesis reaction. The WO species were anchored onto the
framework of MCM-41 by one-step precipitation method, which
can improve the dispersion of tungsten active species and thus
might have a signicant impact on the catalytic activity and
selectivity. The effects of stirring time and Si/W ratio on the
catalyst structures and corresponding reaction performances
were systematically investigated. Moreover, the effect of
upstream isomerization catalyst on the improvement in the
catalytic activity and selectivity was also studied.
2.3 Catalytic evaluation
The catalytic experiments of W-based catalysts for the metath-
esis reaction of ethylene and 1-butene to propene were carried
out in a xed bed reactor. 0.5 g of shaped catalyst (40–60 mesh)
2
. Experimental methods
was placed at the center of reactor. The W-based catalysts need
2.1 Catalyst preparation
ꢀ
to be pretreated at 550 C and 1 bar for 4 h in pure N
2
. The ow
ꢁ1
The W-MCM-41(z) catalyst was prepared by a one-step precipi- rate of pure N
2
was 35 mL min . Aer pretreatment and
31
ꢀ
tation method, where z represents the Si/W molar ratio. cooling to designed reaction temperature, for example 450 C,
Taking W-MCM-41(50) as an example, the typical preparation the mixed feed gas (ethylene/1-butene ¼ 2/1 in molar ratio) was
procedure was as follows: (1) 0.625 g (2.5 mmol) of tungstic acid fed into the catalyst bed. The gas weight hourly space velocity
(
H
2
WO
4
) was added into 5 mL of 15 wt% aqueous H
2
O
2
, and (WHSV), dened as the weight ratio of mixed gas to catalyst
ꢀ
ꢁ1
then the mixed solution was stirred at 60 C for 1 h to generate packed, was regulated at 1.8 h . Two mass ow controllers
soluble low-condensed peroxo species. Aer that, the solution (MFC, Brooks, model 5800E) controlled the ow rates of
was cooled to room temperature and centrifuged to remove ethylene and 1-butene, respectively. All the products were
undissolved tungstic acid. The resulted solution was labelled as analyzed online by a gas chromatograph (Agilent, GC7820A)
S1. (2) 5.467 g (15 mmol) CTAB powder, 240 mL deionized water with a HP-plot/Al O KCl column using a ame ionization
2 3
ꢁ
1
and 92 mL 12.5 mol L hydrochloric acid solution were mixed detector (FID). The 1-butene conversion and products selectivity
to obtain a transparent solution. 28 mL (0.125 mol) of TEOS was were calculated according to the equations reported in our
3
0
mixed with the obtained solution, and solution S1 was imme- previous study.
diately added, gradually forming the precipitate. (3) The ob-
tained reaction mixture was stirred at room temperature for 5 h
3
. Results and discussion
.1 Inuence of stirring time on the catalytic performances
(dened as the stirring time and various stirring time designed
3
to investigate its effect on the catalyst structure), and then the
mixture underwent vacuum ltration and washing by deionized
of W-MCM-41 catalyst
water until the pH reached neutral, obtaining a light yellow Table 1 shows the BET surface area, pore volume and pore size
ꢀ
solid which was then dried overnight at 40 C. (4) The dried of the as prepared W-MCM-41 catalysts with different stirring
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time. It can be seen that the BET surface area rstly increases
and then decreases as the increase of stirring time from 1 to
2
0 h, and the sample obtained with 10 h stirring time shows
2
ꢁ1
both the largest BET surface area of 970 m g and the largest
pore volume of 0.61 m g . As shown in Fig. 1a, the prepared
catalysts are typical mesoporous materials which display
3
ꢁ1
1
obvious type IV isotherms with very clear H -hysteresis loops as
32
the increase of stirring time up to 5 h. As indicated by the pore
size distribution of these samples in Fig. 1b, the pore size is
mainly at about 2 nm when the stirring time is below 5 h. With
an increase in the stirring time, some larger pores of about 4 nm
appear and the amount of these larger pores increases. In
general, the surface area, pore size and pore volume are obvi-
ously inuenced by the stirring time.
The UV-vis DR spectra can provide reliable information
about the local molecular coordination and bonding of W
species. As shown in Fig. 2, all the samples show a typical peak
at ꢂ220 nm and a weaker peak at ꢂ270 nm indicating the
presence of the isolated tetrahedral structure and the oligo-
33
meric octahedral structure of WOx species, respectively.
Moreover, the peak at ꢂ400 nm assigned to the d–d band of
31
crystalline WO
results suggest that the WO
surface of support and no crystalline WO
3
is not observed in all the samples. These
species are highly dispersed on the
is formed.
x
3
Raman spectra were further carried out to reveal the struc-
tural properties of these samples. As shown in Fig. 3, the
ꢁ1
vibrational bands at 493 and 608 cm are attributed to the D2
and D1 defect modes related to tri- and tetra-cyclosiloxane rings
34
ꢁ1
Fig. 1 N2 adsorption–desorption isotherms (a) and pore size distri-
bution (b) of the W-MCM-41(50) catalysts with different stirring time.
The isotherms have been moved verticality to avoid the overlap of
isotherms.
of the support, respectively. The 970 cm Raman band arises
35
from the terminal vs. (W]O) of surface WO
x
.
The band
intensity of support rstly decreases and then increases as the
increase of stirring time and the maximum intensity is obtained
with stirring time of 10 h. Combined with the BET results, it's
deduced that the good crystallinity of the support favors the
high surface area. There is slight inuence of stirring time on
the peak intensity of W]O bond in the sample as the stirring
time of 5 h shows a slight higher peak which indicates the
some larger pores at about 4 nm are formed at a longer stirring
time over 10 h. The catalyst with stirring time of 5 h shows the
highest 1-butene conversion of 88.3% and propene selectivity of
70.4% (Table 2). The higher conversion and selectivity over the
x
presence of more surface WO species.
catalyst with stirring time of 5 h is assigned to the higher
These samples were tested for the metathesis reaction of 1-
butene and ethylene to propene and the results are shown in
Fig. 4 and Table 2. It can be seen from Fig. 4a that all the
catalysts show stable 1-butene conversion aer reaction for 3 h
except the cases with stirring time longer over 10 h which show
slight deactivation with time on stream. As shown in Fig. 1b,
amount of WO species with the isolated tetrahedral structure
x
and W]O bond (Fig. 2 and 3). The detailed relationship
between catalyst structure and catalytic performances will be
discussed later. Thus, a suitable stirring time should be selected
to prepare the excellent metathesis catalyst for the efficient
conversion of ethylene and 1-butene to propene. In the
Table 1 Textural properties of W-MCM-41(50) catalyst with different stirring time
Stirring time
h)
BET surface area
2
ꢁ1
a
3
ꢁ1
a
(
(m g
)
Pore volume (cm g
)
Pore size (nm)
1
5
1
1
2
703
879
970
860
847
0.44
0.44
0.61
0.53
0.57
2.60
2.38
2.60
2.62
2.80
0
5
0
a
Evaluated by the BJH method.
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Fig. 2 UV-vis DRS spectra of the W-MCM-41(50) catalysts with
different stirring time.
following part, the stirring time is assigned to 5 h for all the
prepared W-MCM-41 catalysts.
3.2 Inuence of Si/W ratio on the catalytic performances of
W-MCM-41 catalyst
As presented above, stirring time could obviously affect the
surface area, dispersion and amount of surface WO
Denitely, the structure and amount of surface WO
x
species.
species
x
seems to play an important role in determining the catalytic
performance. In order to further conrm this effect, W-MCM-41
catalysts were prepared with different Si/W ratio since the
varied W loadings may remarkably lead to the changed support
Fig. 4 Time-dependence of the 1-butene conversion (a) and propene
selectivity (b) over W-MCM-41(50) catalyst with different stirring time.
structures and the different amount and existing state of WO
x
species.
2 2
increases probably due to the addition of H O which is used to
Table 3 shows the BET surface area, pore volume and pore
size of the W-MCM-41 catalysts prepared with different Si/W
ratio. It is clear that the sample without the addition of tung-
dissolve tungstic acid and has the effect of the enlargement of
pore volume. The amount of W loading is tested by both XPS
31
and ICP-OES and the results are also listed in Table 3. As
characterized by the ICP-OES, the amount of W loading
increases from 2.6% to 27.0% as the ratio of Si/W decreases
from 100 to 10. The amount of W loading detected by XPS is
always lower than the value by ICP-OES. This discrepancy might
be resulted from that the result of ICP-OES represents the total
amount of W in the catalyst while that of XPS represents the
surface amount of W on the catalyst.
2
stic acid shows the largest BET surface area of up to 1111 m
ꢁ1
g
. When the Si/W ratio is 70 and 100, there is no obvious
change in the BET surface area but the pore volume slightly
As shown in Fig. 5a, all the samples show type IV isotherms
1
with H -hysteresis loops suggesting ordered mesoporous
12
channels. Especially, for the W-MCM-41(10), the hysteresis
loop is much bigger than other samples. As displayed in Fig. 5b,
the introduction of W species also increases the pore size of the
sample by increasing the amount of pores around 3.5 nm. Over
W-MCM-41(10), the amount of pores at about 2 nm is much
lower than the other samples while the amount of pores at
around 3.5 nm is much higher, which indicates a destroyed
zeolite structure over the sample with too much loading of W.
TEM images can clearly reect the morphology of samples.
As shown in Fig. 6, all the samples except W-MCM-41(10) show
ordered two dimensional pore structures assigned to the MCM-
41 zeolite. Moreover, the pore structures of W-MCM-41(30) are
Fig. 3 Raman spectra of the W-MCM-41(50) catalysts with different
stirring time.
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a
Table 2 Catalytic performances of W-MCM-41(50) catalyst with different stirring time
Selectivity (%)
Stirring time
h)
Conversion
(%)
(
Propene
C
5+
trans-2-Butene
cis-2-Butene
Iso-2-butene
1
5
1
1
2
88.1
88.3
82.7
84.8
86.3
69.5
70.4
62.2
65.5
67.5
3.9
4.1
4.8
5.1
5.2
15.1
14.5
18.8
16.8
15.6
11.0
10.5
14.0
12.4
11.4
0.5
0.5
0.2
0.2
0.3
0
5
0
a
Reaction condition: T ¼ 450 C, P ¼ 0.1 MPa, catalyst weight ¼ 0.5 g, WHSV (E + B) ¼ 1.8 hꢁ1, n(E)/n(B) ¼ 2.
ꢀ
not as obvious as the other samples indicating a less ordered
To further characterize the structure and phase properties,
pore structures of the sample. The W-MCM-41(10) with the low-angle and wide-angle XRD patterns of these samples are
highest content of W shows no regular pore structure and the collected and shown in Fig. 8. As shown in Fig. 8a, all the
presence of amorphous SiO . Most of WO species are highly samples except W-MCM-41(10) and W-MCM-41(30) exhibit well-
2 x
dispersed in the MCM-41 zeolite with a very small particle size dened (100) reection indexed on the hexagonal lattice, sug-
of 1–2 nm. However, on the catalyst surface of higher W loading, gesting the formation of the long-range ordered MCM-41 mes-
36
such as W-MCM-41(10) and W-MCM-41(30) sample, some large ophase in these samples. Furthermore, the sample with higher
WO particles can also be observed. Especially for the W-MCM- Si/W ratio exhibits higher crystallinity than the one with lower
x
4
1(10), large amount of WOx particles is observed and the Si/W ratio. This may be due to the differences between Si and W
˚
particle size is over 100 nm. Too much amount of W is indeed atoms in atomic radius (1.32 versus 1.41 A), polarizability and
37
destroying the structure of the expected MCM-41 zeolite, which possible coordination valancy. The W–O–Si likage would
is not suitable for obtaining the well dispersed WO species. For distort the vicinal tetrahedral SiO units. Thus, the high amount
the 18W/MCM-41 sample, the pore structures of MCM-41 is well of W is detrimental for the formation of long-range ordered
reserved, but there are only large WO particles unlike the W- pores. Although the (100) reection of W-MCM-41(30) is not
MCM-41(30) catalyst showing highly dispersed WO species obvious in the XRD patterns, there are still mesopore structures
on the catalyst surface. The one-step prepared method highly can be observed in the TEM images (Fig. 6b). It indicates that
increased the dispersion of WO species. ordered mesopores but not long-range ordered mesopores are
The information about the local molecular coordination and existed in this sample.
x
4
x
x
x
bonding of W species is also detected by UV-vis DR spectra and
Fig. 8b shows the wide-angle XRD patterns of these samples
the results are shown in Fig. 7. The peak intensity at ꢂ220 nm to reveal the dispersion of WO
x
species. The diffraction peaks
3
for W-MCM-41(100) is much stronger than the peak intensity at assigned to crystal WO (PDF#72-1465) appear when the Si/W
ꢂ270 nm, suggesting that most of the W species exist in terms ratio is 30, which are located at 23.1, 23.6, 24.3, 28.9, 33.3 and
ꢀ
of tetrahedral structure. The amount of octahedral structure at 34.1 . The intensity of peaks assigned to WO
3
becomes stronger
ꢂ270 nm increases as the increase of the amount of W. More- as the Si/W ratio is 10 indicating the less dispersion of W
over, the peak at ꢂ400 nm assigned to crystalline WO appears species on the W-MCM-41(10) catalyst. Moreover, the 18W/
3
when the ratio of Si/W is 30 and the peak intensity increases as MCM-41 catalyst shows more enhanced intensity of WO3
the Si/W ratio further decreases to 10. It indicates that the WO
species are highly dispersed on the catalyst surface except the of WO
W-MCM-41(30) and W-MCM-41(10), which is in accordance TEM results. The Raman spectra in Fig. 9 also shows that only
with the TEM results (Fig. 6). characteristic peaks assigned to MCM-41 can be observed when
x
compared with W-MCM-41(30) suggesting the poor dispersion
x
species of 18W/MCM-4, which is consistent with the
Table 3 Textural properties of W-MCM-41 catalysts with different Si/W molar ratio
W (wt%)
BET surface
area (m g
2
ꢁ1
a
3
ꢁ1
a
Sample
By ICP-OES
By XPS
)
Pore volume (cm g
)
Pore size (nm)
MCM-41
0
0
1111
1037
1096
879
654
499
0.53
0.61
0.62
0.44
0.43
0.43
2.17
2.45
2.36
2.38
2.56
3.34
W-MCM-41(100)
W-MCM-41(70)
W-MCM-41(50)
W-MCM-41(30)
W-MCM-41(10)
2.6
4.5
11.9
18.1
27.0
2.1
3.9
6.2
11.0
21.0
a
Evaluated by the BJH method.
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the Si/W ratio is higher than 30 suggesting the highly
dispersed W species on the support surface. If the Si/W ratio
ꢁ
1
decreases to 30, the peaks located at 274, 705 and 815 cm can
be observed assigned to the vibration of crystal WO , which is
x
the deformation vibration mode of W–O–W, bending vibration
mode of W–O, and symmetric stretching vibration mode of W–
38
O, respectively. These results are well in agreement with the
XRD results.
The catalytic performances of these samples are shown in
Table 4. It is obvious that both the 1-butene conversion and the
propene selectivity increase as the decrease of Si/W ratio from
100 to 30 and then both of them decrease slightly as the further
decrease of Si/W ratio from 30 to 10. Therefore, the highest 1-
butene conversion of 92.7% and propene selectivity of 80.8% is
obtained over W-MCM-41(30) catalyst. Interestingly, the 18W/
MCM-41 prepared by the conventional impregnation method
with the same loading of W as the W-MCM-41(30) shows notably
lower 1-butene conversion of 76.5% and propene selectivity of
34.1%.
In general, the metathesis reaction of 1-butene and ethylene
to propene evolves three steps including (1) fast isomerization
of 1-butene, (2) carbene formation and (3) metathesis reaction.
The isomerization is considered to occur on the Si–OH of MCM-
41 support which might act as a weak Brønsted acid site for the
isomerization of C]C bond (Scheme 1), but lack of the ability of
39
metathesis. The followed two steps involving the carbene
formation and the metathesis reaction take place on the highly
Fig. 5
N
2
adsorption–desorption isotherms (a) and pore size distri- dispersed WO species with tetrahedral structure because the
x
bution (b) of the W-MCM-41 catalysts with different Si/W molar ratio. crystalline WO
displays a negligible activity for 1-butene
3
The isotherms have been moved verticality to avoid the overlap of
30
isomerization and inactive in metathesis reaction (Scheme 1).
isotherms.
Compared with the rst step, the followed two steps are very
Fig. 6 TEM images of (a
1
and a
2 1 2 1 2 1 2 1 2
) W-MCM-41(10), (b and b ) W-MCM-41(30), (c and c ) W-MCM-41(50), (d and d ) W-MCM-41(70), (e and e )
1 2
W-MCM-41(100), (f and f ) 18W/MCM-41 catalysts.
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butene conversion is high, the propene selectivity is low and it
gradually increases with time on stream at the initial one hour
(
Fig. 4).
For the one step synthesized W-MCM-41 catalysts, most of
the WO species are highly dispersed and the higher Si/W ratio
x
corresponds to lower W loading. Although the rst isomeriza-
tion step is not rate-determining for the abundant Si–OH on the
MCM-41 support, the followed two steps can be limited for the
x
lack of WO species on the catalyst of higher Si/W ratio. Since
the metathesis reaction is a kind of tandem reaction, the sup-
pressing of the second and third steps will hinder the rst step
to continue. Thus, the conversion of 1-butene should be lower
as the W-MCM-41(100) catalyst shows a very lower conversion of
59.2%. It is not hard to understand that the insufficient WO
x
species is also not favorable for the metathesis reaction of 2-
butene formed from the rst isomerization step and ethylene,
which thereby leads to a lower selectivity to metathesis product
propene but a higher selectivity to isomerization product, 2-
butene. Indeed, the propene selectivity is only 23% and the 2-
butene selectivity is higher than 70% over the W-MCM-41(100)
Fig. 7 UV-vis DRS spectra of 18W/MCM-41 and W-MCM-41 catalysts
with different Si/W molar ratio.
slow, which is accordance with our catalytic results in Fig. 4. At
the very beginning, the fast formed 2-butene can not be
immediately converted into propene for the lack of carbene. The
carbene formation takes time, which determines the formation
of propene. Therefore, at the very beginning, although the 1-
catalyst. As the Si/W ratio decreases lower to 10, WO
is prone to form inactive crystal WO due to the higher W
content, smaller surface area and larger pore size of the
synthesized amorphous SiO rather than the highly ordered
x
species
3
2
MCM-41 (Fig. 6, 8, 9 and Table 3). Therefore, both the 1-butene
conversion and propene selectivity slightly decrease as the Si/W
ratio decreases from 30 to 10. Over the impregnated 18W/MCM-
4
1 catalyst, the dispersion of WO species is much less than the
x
one step preparation method (Scheme 1), which results in less
active sites for the second and third step similarly as the case of
W/MCM-41(10). It is reasonable that 18W/MCM-41 catalyst
exhibits quite lower 1-butene conversion of 76.5% and propene
selectivity of 34.1%. In general, the one step precipitation
method is an efficient approach to obtain the highly dispersed
WOx species with desired tetrahedral structure on MCM-41
support, which is responsible for the higher 1-butene conver-
sion and propene selectivity due to the enhanced metathesis
reaction of 2-butene and ethylene (Scheme 1).
Fig. 8 Low-angle XRD patterns (a) and wide-angle XRD patterns (b) of
the 18W/MCM-41 and W-MCM-41 catalysts with different Si/W molar Fig. 9 Raman spectra of the W-MCM-41 catalysts with different Si/W
ratio. molar ratio.
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a
Table 4 Catalytic performances of W-MCM-41 catalysts with different Si/W molar ratio
Selectivity (%)
Conversion
Sample
(%)
Propene
C
5+
trans-2-Butene
cis-2-Butene
Iso-2-butene
W-MCM-41(100)
W-MCM-41(70)
W-MCM-41(50)
W-MCM-41(30)
59.2
84.6
88.3
92.7
76.5
91.7
23.0
63.7
70.4
80.8
34.1
78.7
3.3
4.5
4.1
3.0
2.6
3.2
39.7
18.3
14.5
8.7
35.6
9.9
33.8
13.3
10.5
6.3
27.3
7.2
0.2
0.2
0.5
1.2
0.4
1.0
1
8W/MCM-41
W-MCM-41(10)
a
Reaction condition: T ¼ 450 C, P ¼ 0.1 MPa, catalyst weight ¼ 0.5 g, WHSV (E + B) ¼ 1.8 hꢁ1, n(E)/n(B) ¼ 2.
ꢀ
Scheme 1 Illustrative drawing of the metathesis reaction of ethylene and 1-butene to propene over W-MCM-41 catalysts prepared by one-step
precipitation method and conventional impregnation method.
3
.3 Inuence of upstream isomerization catalysts on the
above part, we have intensively discussed the effect of W phase
on the reaction performances based on the catalyst character-
ization and catalytic experiments. Herein, it is planned to
improve the catalytic performances by introducing an upstream
catalyst onto the W-MCM-41 catalyst as the upstream catalyst
can pre-isomerize the 1-butene into 2-butene and thereby
propel metathesis reaction. According to this concept, the
upstream catalysts are designed as W-MCM-41(30), MgO and
SBA-15, respectively, and the corresponding results are listed in
Table 5. Among the three upstream catalysts, the MgO shows
catalyst performances of W-MCM-41 catalyst
Since the metathesis reaction for conversion of 1-butene and
ethylene is a kind of the tandem reaction, it is easy to under-
stand that the overall reaction performances can be efficiently
improved by accelerating each step in the reactions. Seen from
the process of metathesis reaction, the strategy for the higher
reaction efficiency can be considered as (1) enhancing the
isomerization rate of 1-butene, (2) promoting the conversion of
x
2-butene and ethylene to propene on the WO species. In the
a
Table 5 Catalytic performances of W-MCM-41(30) catalysts with different upstream isomerization catalysts
Selectivity (%)
Conversion
Catalyst loading
(%)
Propene
C
5+
trans-2-Butene
cis-2-Butene
Iso-2-butene
No upstream catalyst + W-MCM-41(30)
W-MCM-41(30) + W-MCM-41(30)
MgO + W-MCM-41(30)
92.7
93.1
93.1
91.4
80.8
81.3
82.7
78.0
3.0
2.9
2.5
3.0
8.7
8.2
8.1
6.3
6.0
5.9
7.5
1.2
1.6
0.8
1.0
SBA-15 + W-MCM-41(30)
10.5
a
Reaction condition: T ¼ 450 C, P ¼ 0.1 MPa, mW-based catalyst ¼ 0.5 g, mupstream catalyst ¼ 0.5 g, WHSV (E + B) (W-based catalyst) ¼ 1.8 hꢁ1, n(E)/n(B) ¼
ꢀ
2
.
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the highest 1-butene conversion of 93.1% and propene selec-
tivity of 82.7%, while the selectivity to iso-butene slightly
decreases. In contrast, SBA-15 catalyst decreases not only the 1-
butene conversion but also the propene selectivity. In spite of
that the upstream catalyst can be considered to play a role of
isomerization and therefore assist the formation of active sites
and subsequent 2-butene conversion, the actual effect on the
reaction performances is limited. This result suggests that the
isomerization of 1-butene to 2-butene is fast on the metathesis
catalyst (Scheme 1), which is not the rate-determining step in
the metathesis reaction of 1-butene and ethylene to propene.
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In conclusion, we have successfully synthesized W-MCM-41
catalysts by a one-step precipitation method and their catalytic
performances are tested in the metathesis reaction of 1-butene
and ethylene to propene. The results show that both stirring time
and Si/W ratio obviously affect the structures of MCM-41 and W
phase as conrmed by N
DRS, XRD and Raman characterization. The catalytic evaluation
shows that the amount and state of WO species directly deter-
mines the 1-butene conversion and propene selectivity as the
isolated tetrahedral WO species is mainly responsible for the
2
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1
carbene formation and subsequent metathesis reaction. More-
over, loading an upstream catalyst with the isomerization ability
of converting 1-butene to 2-butene could not signicantly
enhance the reaction performance, which might be due to the
fast isomerization of 1-butene to 2-butene and relatively slow
metathesis reaction of 2-butene and ethylene to propene on the
investigated W-MCM-41 catalysts.
1
1
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2
2
2
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2
1 J. F. Wu, A. Ramanathan, W. K. Snavely, H. Zhu, A. Rokicki
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There are no conicts to declare.
Acknowledgements
This work is supported by the National Natural Science Foun-
dation of China (21706098, 21576119, 21878127), the Natural
Science Foundation of Jiangsu Province (BK20151125), the China
Postdoctoral Science Foundation (2016M601714), the Funda-
mental Research Funds for the Central Universities
2
014, 50, 7250–7253.
(JUSRP51720B, JUSRP11813), and the Programme of Introducing
ˇ
6 H. Balcar and J. Cejka, Coord. Chem. Rev., 2013, 257, 3107–
Talents of Discipline to Universities (111 Project B13025). We also
appreciate the support from the Central Laboratory of School of
Chemical and Material Engineering of Jiangnan University.
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W. M. Yang, Z. C. Liu and W. Xu, Mater. Lett., 2006, 60,
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8 C. Lin, K. Tao, H. B. Yu, D. Y. Hua and S. H. Zhou, Catal. Sci.
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