Differences in acid and catalytic properties of W incorporated spherical SiO2 and 1%Al-doped SiO2 in propene metathesis

Article abstract of DOI:10.1016/j.cattod.2020.06.046

Propene self-metathesis to ethylene and 2-butene was investigated over tungsten-based catalysts (9 wt.% W) supported on silica and 1%Al-doped SiO₂, prepared by different processes. On pure SiO₂, incorporating WO_x improved tungsten dispersion and formation of active tetrahedral WO_x species, compared to the impregnated ones. However, negative effect was found when tungsten was incorporated in the 1%Al-doped SiO₂ as the catalysts contained more crystalline WO_x, lower tetrahedral species, and lower Lewis acid sites, compared to the W-imp 1Al₂O₃/SiO₂ prepared by impregnation. Incorporating both W and Al in the SiO₂ by sol-gel method may lead to suppression of bridge hydroxyl Si(OH)Al group, as a consequence more crystalline WO_x was formed. Nevertheless, the abundance of Bronsted and Lewis acid was found to play important role in the propene metathesis and the secondary metathesis of 1-butene reactions than surface silanol and the presence of tetrahedral tungsten oxide species on the 1%Al-doped SiO₂ supported catalysts.

Full text of DOI:10.1016/j.cattod.2020.06.046

CatalysisTodayxxx(xxxx)xxx–xxx
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Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Differences in acid and catalytic properties of W incorporated spherical SiO2
and 1%Al-doped SiO2 in propene metathesis
Nattaphon Hongrutaia, Sutasinee Watmaneea, Tanaya Srisakwattanaa, Sippakorn Wannakaob,
Piyasan Praserthdama, Joongjai Panpranota,*
a Center of Excellence on Catalysis and Catalytic Reaction Engineering, Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok,
10330, Thailand
b SCG Chemicals, Co., Ltd., 1 Siam-cement Rd, Bang Sue, Bangkok, 10800, Thailand
A R T I C L E I N F O
A B S T R A C T
Keywords:
Surface silanol
Dispersion
Bronsted and Lewis acid
Active sites
Metathesis
Propene self-metathesis to ethylene and 2-butene was investigated over tungsten-based catalysts (9 wt.% W)
supported on silica and 1%Al-doped SiO2, prepared by different processes. On pure SiO2, incorporating WOx
improved tungsten dispersion and formation of active tetrahedral WOx species, compared to the impregnated
ones. However, negative effect was found when tungsten was incorporated in the 1%Al-doped SiO2 as the
catalysts contained more crystalline WOx, lower tetrahedral species, and lower Lewis acid sites, compared to the
W-imp 1Al2O3/SiO2 prepared by impregnation. Incorporating both W and Al in the SiO2 by sol-gel method may
lead to suppression of bridge hydroxyl Si(OH)Al group, as a consequence more crystalline WOx was formed.
Nevertheless, the abundance of Bronsted and Lewis acid was found to play important role in the propene me-
tathesis and the secondary metathesis of 1-butene reactions than surface silanol and the presence of tetrahedral
tungsten oxide species on the 1%Al-doped SiO2 supported catalysts.
1. Introduction
Compared to Mo- [6–8] and Re-based catalysts [9–11], W-based
catalysts [3,12–15] are more attractive and effective for metathesis
Light olefins such as ethylene, propylene and butene are important
raw materials for many industrial chemicals. Moreover, the estimated
consumption of these olefins gradually increases every year but they
usually encounter uncertain market prices. Propene self-metathesis has
attracted attention as an important reaction in supporting the global
demand of these olefins. Metathesis reaction has an excellent product
selectivity, uses less energy consumption (temperature 300–600 ◦C)
with lower CO2 emission than other technology processes such as steam
cracking of naphtha and fluid-catalytic cracking [1]. For industrial
applications, both heterogeneous and homogeneous metathesis cata-
lysts have been employed. Generally, homogeneous catalysts show high
activity and selectivity but they encounter limitation due to catalyst
recovery and product separation issues. A heterogeneous silica sup-
ported WOx catalyst that operates at 400 − 600 °C has been successfully
employed in the large scale olefins production such as the Olefins
Conversion Technology (OCT) process licensed by the Lummus ABB
[2]. Development of new efficient tungsten-based catalysts for me-
tathesis has been continuingly reported [3–5] however structure −
activity/selectivity relationship of these catalysts has still not been well
established.
owing to their high activity at moderate temperature and high stability
upon regeneration [15–19]. The excellent performances in metathesis
were attributed to the highly dispersed tungsten oxide in the form of
tetrahedral species and proper reducibility of WOx species [20,21].
Conventional tungsten catalysts have some drawbacks such as the
presence of less active sites, inappropriate acidity, and unsteadiness of
surface properties [16,22]. Improvement in the tungsten catalyst per-
formances was achieved by changing the support types [16,23], in-
creasing the percentage of tungsten loading [21,24], and modification
of the synthesize methods [15,25].
The property of supported metal oxide catalysts is obviously de-
pendent on the nature of the support and the synthesis method.
Different types of supports including silica [4,26], alumina [27], and
silica-alumina [16,23,25] are usually employed as the supports for
tungsten-based catalysts. Although silica supported tungsten catalysts
are well known to be highly active and stable [28], low acidic silica
supports led to a long induction period due to less amount of active
tungsten carbene. The strong acidity and interaction of support and
tungsten have been reported on alumina supported tungsten catalysts,
which can promote dispersion of tungsten oxide but they favored side
⁎ Corresponding author.
E-mail address: joongjai.p@chula.ac.th (J. Panpranot).
https://doi.org/10.1016/j.cattod.2020.06.046
Received 21 February 2020; Received in revised form 14 May 2020; Accepted 17 June 2020
0920-5861/©2020PublishedbyElsevierB.V.
Pleasecitethisarticleas:NattaphonHongrutai,etal.,CatalysisToday,https://doi.org/10.1016/j.cattod.2020.06.046

N. Hongrutai, et al.
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reactions such as secondary metathesis and isomerization. Accordingly,
silica-alumina supports have been suggested to provide a higher dis-
persion of tungsten oxide. The presence of WeOH bonds by acidic sites
was found to encourage the higher formation of tungsten carbene [25].
Nevertheless, the acidity tended to increase with increasing alumina
content especially the Lewis acid sites [29].. Liu N. et al. [23] reported
that tungsten oxide loaded on silica-alumina exhibited higher acidity
and excellent catalytic activity compared to pure silica and pure alu-
mina supported ones for 1-butene metathesis. Because the acidic sup-
port might contribute to rapid deactivation of catalysts, moderate dis-
persion and acidity of the support may be desirable.
The characteristics of catalysts also depended on the synthesis
method. Specific surface area, pore size, and pore volume can be en-
hanced by using the sol-gel technique [25,30]. The forming spherical
particle demonstrated the outstanding mechanical strength, excellent
thermal stability, and promoting the efficiency of the surface properties
on the mixed oxide [30]. Furthermore, improved tungsten catalyst
performance has been obtained by the incorporation of tungsten metal
in support structure [31,32]. As reported in the literature, the spherical
silica incorporating tungsten catalysts prepared by the one-pot aerosol-
assisted sol-gel process showed good performances in the cross-me-
tathesis of ethylene and trans-2-butene owing to their excellent texture,
precise metal-composition, and well propene selectivity [3].
In this study, spherical silica and 1% Al-doped SiO2 supported
tungsten catalysts were employed as the catalyst supports for pre-
paration of 9 wt.% W by different methods and tested in the propene
self-metathesis doped. The performances of tungsten catalysts were
correlated to the dispersion of WOx species, the acidity of Bronsted and
Lewis acid sites, the catalysts structure, the formation of active tungsten
carbene, and the surface properties of the catalysts. The higher dis-
persion and appropriate acidic amount on 1% Al-doped SiO2 was ex-
pected to provide better propene conversion and ethylene/butene se-
lectivity.
silica gel (Davisil grade 646) and aluminum nitrate nonahydrate (98 %,
Aldrich). These catalysts are denoted as “W-imp SiO2(gel)” and “W-imp
xAl2O3/SiO2(gel)” where x represents a percentage of Al2O3 in the
support. For spherical supported tungsten catalysts (SiO2(SP) and
xAl2O3-SiO2(SP)), these catalysts are denoted as “W-imp SiO2 (SP)” and
“W-imp xAl2O3-SiO2 (SP)”.
2.3. Preparation of spherical tungsten oxides catalysts
The tungsten incorporated spherical silica catalysts were synthe-
sized via a sol-gel technique. The ammonium metatungstate hydrate
and TEOS including aluminum nitrate nonahydrate for silica-alumina
(0.007 W: 1 TEOS: 1 Al(NO3)3·9H2O by mol) were gradually doped
simultaneously in the mixed solution of ethanol, deionized water, and
ammonia to gain the gel composition. After that, the catalysts were
dried overnight and calcined in air for 8 h at 550 ◦C with a heating rate
2 ◦C/min. These catalysts are denoted as “W-SiO2(SP)” and “W-xAl2O3-
SiO2(SP)” where x represents a percentage of Al2O3.
2.4. Reaction tests
Propene self-metathesis reaction was carried out in a tubular
stainless-steel fixed bed reactor (IDtube =17 mm) at 550 ◦C and atmo-
spheric pressure. In a typical reaction test, 1 g of tungsten catalyst was
placed in an isothermal zone of the reactor. Firstly, the catalysts were
reduced with high purity N2 flow (50 cm3/min) at 500 ◦C for 1 h, then
the temperature was ramped to the reaction temperature at 550 ◦C
under flowing N2. The feed stream containing 20 % propene balance N2
as fed to the reactor (WHSV=4.08 h−1). The exhausted gases were
analyzed using an online gas chromatography (Agilent 7820) with an
FID detector. The propene conversion and the product selectivity were
determined by Eqs. 1 and 2.
C3H6in − C3H6out
Propene Conversion (%) =
x 100
C3H6in
(1)
2. Materials and methods
P
iout
Component i Selectivity (%) =
x 100
2.1. Preparation of spherical particles silica and silica-alumina supports
C3H6in − C3H6out
(2)
where C3H6in and C3H6out are the quantity of propene content in feed
and exhaust gas, respectively. Piout indicated the content of component i
in the effluent gas.
The bare spherical particle support was synthesized via a sol-gel
technique using cetyl trimethyl ammonium bromide (CTAB) as a
structure directing agent (98 %, Sigma). Firstly, CTAB was dissolved in
the solution of ethanol (99.99 %, RCI Labscan), deionized water, and
ammonia (30 %wt., AppliChem Panreac) under stirring rate 350 rpm at
the room temperature. The composition of CTAB, ethanol, DI water,
and ammonia are as follows: 0.3, 58, 114, and 11 by mol. Then, tet-
raethoxysilane (98 %, Aldrich) as a silica precursor and aluminum ni-
trate nonahydrate (98 %, Aldrich) as an alumina precursor were slowly
doped into the solution to generate gel composition as 1 TEOS and 1
TEOS: 1 Al(NO3)3·9H2O by mol for silica and silica-alumina support,
respectively. The solution was further stirred for 2 h and the precipitate
was eliminated by filtration and washing with DI water until becoming
neutral. After that, the bare supports were dried in an oven at 110 ◦C
for overnight and calcined in air at 550◦C with a heating rate 2◦C/min
for 6 h to remove the surfactant. These supports were labeled as
“SiO2(SP)” and “xAl2O3-SiO2(SP)” where x represents a percentage of
Al2O3 in the support.
2.5. Catalyst characterization
N2 adsorption-desorption technique was used to classify the specific
surface area, pore volume, and average mean pore diameter of the
catalysts. The samples were degassed under vacuum at 160 ◦C for 4 h
before performing under N2 physisorption at -196 ◦C. The specific
surface area (SBET) was determined by the Brunauer-Emmett-Teller
(BET) method and the pore volume and mean pore diameter were
calculated by the Barreet-Joyner-Halenda (BJH) method.
The scanning electron microscope (SEM) was investigated by using
a Hitachi S-3400 N model for determining the surface spherical shape
and morphology. The X-ray diffraction (XRD) technique was used to
determine the crystalline structure of the catalysts. XRD patterns were
obtained with a D8 Advance of Bruker AXS using CuKα radiation with a
Ni filter to observed in the 2θ range from 20° to 80° at a step size 0.02
2.2. Preparation of tungsten oxides catalysts
s
−1. The surface structure of tungsten oxide species was observed by
UV–vis spectroscopy (UV–vis) using Lambda 650 spectrophotometer.
Raman spectra were decided to study the molecular structure of the
crystalline and tetrahedral phase of tungsten oxide species that were
carried out on a Dispersive Raman Microscope (Bruker Optics, Senterra)
at room temperature using the Laser excitation wavelength at 532 nm
with laser output 20 mW and a TE-Cooled CCD detector.
The tungsten oxides catalysts with 9 wt.% W were prepared by in-
cipient wetness impregnation method with an aqueous solution of
ammonium metatungstate hydrate (99.9 %, Aldrich) as the tungsten
precursor. The impregnated catalyst was kept at room temperature for
2 h and dried at 110 ◦C for overnight. Subsequently, the dried sample
was calcination in air at 550 ◦C for 8 h with a heating rate 10 ◦C/min.
For commercial catalysts, silica and silica-alumina were prepared by
The total acidity and acid strength of the catalysts was observed by
temperature-programmed desorption of ammonia (NH3-TPD) via
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Micromeritic Chemisorb 2750 automated system. Firstly, 0.1 g of the
fresh catalyst was packed in a quartz U-tube reactor. Subsequently, the
catalyst was pretreated with a He flow (25 mL/min) at 500 ◦C for 1 h
and then cooled down to 50 ◦C in flowing He. Afterward, the pretreated
catalyst was adsorbed NH3 on the acid sites with 15 % NH3/He (25 mL/
min) at 50 ◦C for 30 min and the catalyst was purged with a He flows
(25 mL/min) for 30 min to remove the excess NH3 or physisorbed NH3.
Finally, the catalyst was linearly heated to the reaction temperature at
550 ◦C (heating rate 10 ◦C /min) and the desorbed ammonia was de-
tected by the TCD detector and Chemisorb system. In-situ NH3-IR were
performed to examine the type of acidity of the catalysts including
Bronsted and Lewis acid sites via a VERTEX 70 model. The catalyst was
pretreated from 40 ◦C to 500 ◦C under N2 flow (10 mL/min) and then it
was cooled down to 40 ◦C for recorded as the dehydrated background
spectrum. Subsequently, the treated sample was adsorbed NH3 with 15
% of NH3/He (10 mL/min) that admitted into the catalyst at 40 ◦C for
30 min. After purging the excess or physisorbed NH3 under N2 flow for
30 min, the IR spectra were collected as the adsorbed of the catalyst.
The hydrogen temperature-programmed reduction (H2-TPR) tech-
nique was used to investigate the reducibility of the catalysts via a
Micrometrics Chemisorbs 2750 automated system. Firstly, the sample
(0.1 g) was preheated with an Ar flow (25 mL/min) from 50 ◦C to 500
◦C for 1 h at atmospheric pressure and then cooled down to 50 ◦C.
Finally, the temperature was increased linearly from 50 °C to 940 °C
under 10 % H2/Ar (15 mL/min) with a ramping rate 10 °C/min.
Hydrogen consumption was obtained by a thermal conductivity de-
tector (TCD). 1H and 29Si MAS NMR spectra were measured on a Bruker
AVANCE III HD (Ascend 400WB) spectrometer with 4 mm MAS probe
head and speed rate 8 kHz.
Table 1
Textural characterization results of the catalysts.
Catalysts
SBET (m2/
g)
Pore volume (VP)
(cm3/g)
Mean pore diameter
(nm)
W-imp SiO2(gel)
W-imp SiO2(SP)
W-SiO2(SP)
W-imp 1Al2O3/
SiO2(gel)
256
797
857
221
0.89
0.61
0.72
0.77
13.55
2.47
2.89
13.54
W-imp 1Al2O3-SiO2(SP)
W-1Al2O3-SiO2(SP)
807
687
0.62
0.64
2.52
3.28
BET surface area around 857 m2/g. In contrast, 1% Al-doped SiO2 with
tungsten incorporated catalysts had lower BET surface area as com-
pared to the bare spherical silica particles, suggesting that addition of
1 wt.% alumina strongly affected the textural properties of the catalyst.
It should also be noted that different heating rates may affect the
structure and properties of the catalysts. A heating rate 2 °C/min was
necessary to ensure formation of spherical particles with and without W
loading. It is an important parameter for controlling the phase transi-
tion within the surfactant templates [33]. For the impregnated ones,
typical heating rate was used for the calcination step.
The wide-angle powder XRD patterns of supported tungsten cata-
lysts are depicted in Fig. 2(A) and (B). The diffraction peaks at 2θ peaks
around 22−34◦ corresponding to the crystalline phase of WO3 [4,12]
were observed. The W-imp SiO2(gel) and W-1Al2O3-SiO2(SP) catalysts
showed strong sharp peaks, suggesting poor tungsten dispersion.
Moreover, the crystalline phase of WO3 was often considered as the
inactive sites of the metathesis reaction [34,35]. Generally, spherical
silica particles support tended to provide better dispersion of the
tungsten especially for the tungsten incorporated spherical silica [15].
1% Al-doped SiO2 was found to significantly affect the surface silanol
structures of the corresponding supported W catalysts but such effect
also depended on the different preparation methods used. The W-imp
1Al2O3-SiO2(SP) apparently exhibited lower XRD intensity of the
crystalline phase of WO3 compared to the W-imp SiO2(SP). It is sug-
gested that amorphous WOx was preferentially formed on the alumina
support and/or tungsten oxides were highly dispersed [36]. However,
W-1Al2O3-SiO2 (SP) demonstrated the highest intensity of the crystal-
line phase of WO3 suggesting the formation of more agglomerated
tungsten oxide species.
3. Results and discussion
3.1. Characteristics of the catalysts
The morphology of the catalysts is disclosed in Fig. 1. The im-
pregnated W-imp SiO2(gel) and W-imp 1Al2O3/SiO2(gel) catalysts
showed irregular crystal with a rough surface. Meanwhile, the other
catalysts exhibited spherical particles with particle size ranging be-
tween 200 and 800 nm. The W-imp spherical silica and tungsten in-
corporated spherical silica catalysts showed identical morphology.
Moreover, there was insignificant change when 1 wt.% alumina was
added to the catalysts. Table 1 summarizes the structural properties of
the catalysts. High BET surface area was obtained on the spherical silica
supported tungsten catalysts and the tungsten incorporated spherical
silica with tungsten incorporated spherical silica exhibited the highest
To further investigate the structure of tungsten species, the UV–vis
spectra were collected on tungsten catalysts and are presented in Fig. 3.
The spectra demonstrate the different structures of tungsten species in
Fig. 1. SEM images of the catalysts: (A) W-imp SiO2(gel), (B) W-imp SiO2 (SP), (C) W-SiO2 (SP), (D) W-imp 1Al2O3/SiO2(gel), (E) W-imp 1Al2O3-SiO2 (SP), and (F) W-
1Al2O3-SiO2 (SP).
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Fig. 2. Wide-angle XRD patterns of tungsten catalysts.
Fig. 3. UV–vis spectra of the catalysts:
W-imp SiO2(gel),
W-imp SiO2
(SP),
W-SiO2 (SP),
W-imp 1Al2O3/SiO2(gel),
W-imp 1Al2O3-SiO2
(SP), and
W-1Al2O3-SiO2 (SP).
which the absorption bands at 210 and 240 nm were attributed to
isolated tetrahedral tungsten oxide species and isolated tetrahedral with
other linkage modes [12]. The W-imp 1Al2O3/SiO2(gel) and W-imp
1Al2O3-SiO2(SP) catalysts exhibited higher isolated tetrahedral tung-
sten oxide species than the other tungsten catalysts. Moreover, the
absorption bands at 270, 340 and 390 indicated the presence of octa-
hedral polytungstate species, normal octahedral species, and distorted
octahedral species, respectively [4,12,21]. The silica and 1% Al-doped
SiO2 impregnated tungsten catalysts displayed a distinct intensity ab-
sorption bands due to the abundance of octahedral species. It is widely
accepted that the octahedral polytungstate species are less active for
metathesis as compared to the isolated tetrahedral tungsten species
[34,37].
Fig. 4. Raman spectra of tungsten catalysts (A) and the isolated tetrahedral
Approximate quantitative and qualitative amounts of crystallite and
tetrahedral tungsten oxide species of the tungsten catalysts were stu-
died via Raman spectra. The Raman spectra of tungsten catalysts are
shown in Fig. 4(A). All the catalysts demonstrated similar results
showing the major vibrational modes at 275, 717, and 810 cm−1
[15,26,38] and the other minor bands were located at 135 and
328 cm−1 [39], which represent the characteristics of the amorphous
and crystalline phase of tungsten oxide species [40]. The W-1Al2O3-
SiO2(SP) catalysts exhibited the highest intensity peaks of WO3 owing
to the presence of the crystalline phase of tungsten oxide species.
Meanwhile, the W-imp SiO2(SP) and W-imp 1Al2O3-SiO2(SP) catalysts
revealed a higher peak at 717 cm-1 than the W-1Al2O3-SiO2(SP) cata-
lyst, suggesting that these catalysts existed plentiful of bending mode of
W]O [39]. As presented in Fig. 4(B), the Raman bands at 970 cm−1
were assigned to the isolated tetrahedral tungsten oxide species. The
tungsten oxide species region at 970 cm-1 (B):
W-imp SiO2(gel),
W-imp 1Al2O3/SiO2(gel), W-imp 1Al2O3-
W-1Al2O3-SiO2 (SP).
W-imp
SiO2 (SP),
W-SiO2 (SP),
SiO2 (SP), and
silica supported tungsten catalysts exhibited small intensity band
whereas a stronger band was observed over W-imp 1Al2O3/SiO2(gel)
and W-imp 1Al2O3-SiO2(SP) catalysts. This can be inferred that these
catalysts contained highly active sites for metathesis reaction [20]. The
Raman results were consistent with the evidence provided by the XRD
and the UV–vis spectra.
3.2. Acidity and surface chemistry
Acidity of the catalysts was monitored by temperature programmed
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from 1630 to 1580 cm−1. Generally, Lewis acid sites are necessary for
the initiation of reactive adsorption with olefin on tungsten oxide
(forming metal carbene) in metathesis [3]. The Bronsted acid sites were
detected at IR band ∼ 1460 and 1700 cm−1 [43]. It is suggested that
the distinctive of Bronsted acid sites appeared on silica-alumina sup-
ported tungsten catalyst especially on the tungsten incorporated sphe-
rical silica catalyst. The presence of Bronsted acid sites led to more side
reactions such as isomerization, secondary metathesis and decreased
the primary metathesis from the formation of octahedral polymeric
structures that affects lower tungsten active sites [44]. The plentiful of
Lewis acid sites was found to depend on the preparation method and
the properties of the support in the following order: W-imp
SiO2(gel) > W-imp SiO2(SP) and W-SiO2(SP) for silica supported
tungsten catalysts and W-1Al2O3-SiO2(SP) > W-imp 1Al2O3-SiO2(SP)
and W-imp 1Al2O3/SiO2(gel) for silica-alumina supported tungsten
catalysts. Moreover, the abundance of Bronsted acid sites was observed
on the spherical supported tungsten catalysts and the tungsten in-
corporated spherical silica catalysts in the following sequence: W-
1Al2O3-SiO2(SP) > W-imp 1Al2O3-SiO2(SP) > W-imp SiO2(SP) > W-
SiO2(SP) > W-imp 1Al2O3/SiO2(gel) and W-imp SiO2(gel). It is sug-
gested that alumina content can increase the acidity especially Bronsted
acid sites resulting in lower Lewis acid sites. The amounts of Lewis and
Bronsted acid sites calculated from the integrated IR band results are
shown in Table 2.
The reducibility of W catalysts and the interaction between the WOx
species and the supports were investigated by the H2-TPR and the re-
sults are shown in Fig. 7. In the temperature range studied (200–950
◦C), the peak at low temperature 450 ◦C was assigned to the reduction
of octahedrally coordinated WO3 species [21]. The W-imp SiO2(gel)
and W-1Al2O3-SiO2(SP) catalysts exhibited the reduction of WO3 in
octahedrally coordinated at moderately high temperatures at 520–560
◦C, which was ascribed to the strong interaction between the W species
and the support. The observed peaks at a high temperature about 800
◦C were attributed to the reduction of micro crystallites to the W metal
(W6+ → W°) or surface amorphous WO3 species [25]. Generally, alu-
mina content in the support can promote the dispersion of the tungsten
catalysts resulting in lower crystalline WOx. Therefore, lower reduction
peaks of the crystalline WOx species to W metal on W-imp 1Al2O3/
SiO2(gel) and W-imp 1Al2O3-SiO2(SP) catalysts were lower than the W-
imp SiO2(gel) and W-imp SiO2(SP) catalysts with the W-1Al2O3-
SiO2(SP) catalysts exhibited the highest intensity of this peak due to the
presence of the highest crystalline phase WO3, consistent with the XRD
results.
Fig. 5. NH3-TPD profiles of tungsten catalysts from 50 ◦C to 500 ◦C after steady
absorption:
W-imp SiO2(gel),
W-imp SiO2 (SP),
W-SiO2 (SP),
W-
imp 1Al2O3/SiO2(gel),
SiO2 (SP), and
W-imp 1Al2O3-SiO2 (SP),
1Al2O3-SiO2 (SP).
W-1Al2O3-SiO2 (SP)),
desorption of ammonia (NH3-TPD) as shown in Fig. 5. In the tem-
perature range studied (50–500 ◦C), the main peak at ∼ 100–120 ◦C,
∼180–220 ◦C, and ∼ 300 ◦C were assigned to weak, medium, and
strong acid site [21,23,41], respectively. It was found that the total
acidity of silica supported tungsten catalyst was less than the 1% Al-
doped SiO2 supported tungsten catalysts as compared to an identical
preparation method. Although silica support is seldom acidic and the
1Al2O3-SiO2 support showed higher acidity, the silica supported tung-
sten catalysts exhibited comparable acidity to the silica-alumina sup-
ported ones. The total acidity and acid strength of supported WOx
catalysts were much higher than the bare SiO2 and 1Al2O3-SiO2 sup-
ports. In other words, it can be said that the acid mainly originates from
the tungsten oxides on these supports, as also found on other supported
tungsten catalyst systems [21,42]. Moreover, the W-imp SiO2(SP) and
W-SiO2(SP) catalysts showed outstanding weak acid sites whereas the
W-imp 1Al2O3-SiO2(SP) and W-1Al2O3-SiO2(SP) catalysts exhibited
notable strong acid sites.
In order to investigate the type of acidity, the tungsten catalysts
were analyzed by IR spectra of chemisorbed NH3 as presented in Fig. 6.
The IR bands at ∼ 1280 and 1630 cm−1 corresponded to the Lewis acid
sites [43]. For the impregnated catalysts, the IR bands slightly shifted
The environment of Si atoms was confirmed via 29Si solid state MAS
NMR technique as depicted in Fig. 8(A). The signal at ∼ -90 (I), -100
(II), and -110 (III) ppm corresponded to germinal silanol group (Si
(OH)2(OSi)2), terminal silanol group (Si(OH)(OSi)3), and siloxanes (Si
(OSi)4) respectively [4]. The results indicate that the silica-alumina
supported tungsten catalysts exhibited higher amount of the silanol Si
(OH)2(OSi)2 structure than the silica supported tungsten catalysts. This
can be explained by that alumina with high acidity especially Bronsted
acid sites directly promoted the changing of the surface silanol structure
to form a higher hydroxyl group. Furthermore, the presence of terminal
silanol Si(OH)(OSi)3 structure tends to increase isolated tetrahedral
tungsten oxide species [4] as also revealed by our Raman results. The
abundance of silanol Si(OSi)4 structure was determined to be in the
following sequence: impregnation method > spherical particle support
and tungsten incorporated spherical silica. The minor signals at ∼ -103
and -108 ppm were assigned to silanol group that attached to other
species W-O-Si(OSi)3 and Si(OSi)3(OAl), respectively [45,46].
To further investigate the surface silanol structure, the 1H MAS
NMR spectra of tungsten catalysts in this study are shown in Fig. 8(B).
The peak position at 1.2 and 2.1 were attributed to isolated silanol Si
(OH) groups at the SiO2 surface [47,48]. The silica supported tungsten
catalysts demonstrated that these silanol structures were more promi-
nent than silica-alumina supported tungsten catalysts due probably to
Fig. 6. In-situ IR spectra of NH3 chemisorbed over tungsten catalysts at 40 ◦C:
W-imp SiO2(gel),
1Al2O3/SiO2(gel),
W-imp SiO2 (SP),
W-imp 1Al2O3-SiO2 (SP), and
W-SiO2 (SP),
W-imp
W-1Al2O3-SiO2 (SP).
5

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Table 2
The amount of Lewis and Bronsted acid sites.
Lewis acid sites (a.u.)
1280
Bronsted acid sites (a.u.)
Total
Lewis
Total Acidity/SBET
Catalysts
1630
1460
1700
Bronsted
W-imp SiO2(gel)
W-imp SiO2(SP)
W-SiO2(SP)
W-imp 1Al2O3/SiO2(gel)
W-imp 1Al2O3-SiO2(SP)
W-1Al2O3-SiO2(SP)
2.01
3.69
7.30
5.91
5.24
3.77
1.50
1.05
1.01
2.22
0.10
0.37
1.08
11.19
8.63
3.61
14.10
23.21
0.46
0.97
1.13
1.37
0.73
1.32
3.51
4.74
8.31
8.13
5.34
4.13
1.54
12.16
9.76
4.98
14.83
24.52
0.020
0.021
0.021
0.059
0.025
0.042
Fig. 7. H2-TPR profiles of tungsten catalysts:
W-imp SiO2(gel),
W-imp
SiO2 (SP),
W-SiO2 (SP),
W-imp 1Al2O3/SiO2(gel),
W-imp 1Al2O3-
SiO2 (SP), and
W-1Al2O3-SiO2 (SP).
the alumina content caused the changing of surface silanol structure.
The peak at 3.5–4.8 ppm was assigned to strongly hydrated surface si-
lanol and the bridged hydroxyl groups of Si and Al (Si(OH)Al) for silica
and silica-alumina supported tungsten catalysts, respectively
[47,49,50]. Although the W-1Al2O3-SiO2(SP) catalyst displayed the
highest Bronsted acid sites, the 1H NMR results indicated the lowest
intensity of the hydroxyl group. Therefore, it can be said that the main
contribution of Bronsted acid sites were from the tungsten oxide spe-
cies, which were consistent with the XRD results (see Scheme 1).
3.3. Catalytic performances of tungsten catalysts and structure-activity
relationship
The widely accepted mechanism for olefin metathesis over hetero-
geneous catalysts has been suggested to be similar to that reported for
homogeneous catalysts, which follows that proposed by Chauvin et al.
[51,52]. Firstly, the reaction starts by binding of alkene to a metallo-
carbene complex to form a metallacyclic intermediate. Then, they de-
compose to release the product alkene and generate new carbene spe-
cies. Many factors have been reported to affect the formation of metal
carbene intermediates such as the Lewis acid site [3] and the formation
of WeOH bonds [25]. However, the performances of supported WOx
systems also depend largely on the structure and dispersion of the pre-
catalytic active forms of the tungsten oxides, i.e., the isolated tetra-
hedral species are typically more active than octahedral polytungstate
species [34,37].
The catalytic performances of the tungsten catalysts in the propene
self-metathesis at 550 ◦C with 20 % propene balanced in N2 under
ambient pressure at WHSV 4.08 h−1 are depicted in Fig. 9. As can be
seen in Fig. 9(A), the initial propene conversion depended on the pre-
paration method and the properties of the support. The initial propene
conversion over the silica supported tungsten catalysts increased in the
Fig. 8. 29Si (A) and 1H MAS NMR (B) spectra of tungsten catalysts:
SiO2(gel), W-imp SiO2 (SP), W-SiO2 (SP), W-imp 1Al2O3/SiO2(gel),
W-imp 1Al2O3-SiO2 (SP), and W-1Al2O3-SiO2 (SP).
W-imp
following preparation method order: impregnation (15 %) < spherical
particle support (29 %) < incorporating spherical W catalyst (37 %).
Moreover, the 1% Al-doped SiO2 supported tungsten catalysts showed
higher initial propene conversion, which could be attributed to the
abundance of the strong acid sites and Bronsted acid sites that led to
side reactions including isomerization and secondary metathesis as
shown in Fig. 9(B). On pure SiO2, incorporating WOx improved tung-
sten dispersion and formation of active tetrahedral WOx species, com-
pared to the impregnated ones. The higher tungsten dispersion in the
form of tetrahedral WOx species was correlated well with the higher
terminal silanol presented. On the other hand, negative effect was
found when tungsten was incorporated in the 1%Al-doped SiO2 as the
catalysts contained more crystalline WOx, lower tetrahedral species,
and lower Lewis acid sites, compared to the W-imp 1Al2O3-SiO2
6

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Scheme 1. Probability of the surface silanol structure on SiO2 and 1%Al-doped SiO2 supported W catalyst.
prepared by impregnation. Incorporating both W and Al in the SiO2 by
sol-gel method may lead to suppression of bridge hydroxyl Si(OH)Al
group, as a consequence more crystalline WOx was formed. The 1% Al-
doped SiO2 supported tungsten prepared by impregnation and in-
corporating of W catalysts exhibited similar propene conversion. Such
results suggest that alumina can promote the metathesis activity to a
certain extent. However, the highest initial conversion 38.5 % was
obtained on W-imp 1Al2O3-SiO2(SP) catalyst due probably to the pre-
sence of tetrahedral tungsten species and less crystalline WOx phase.
Subsequently, the conversion increased dramatically and gradually
reached steady-state in 3 h time-on-stream. It is believed that these
catalysts, particularly incorporating spherical W catalysts, can reduce
the induction period of the propene metathesis. The highest final con-
version was obtained with the catalysts in the following sequence: W-
literature as given in Table 3.
According to the ethylene selectivity as shown in Fig. 9(C), an in-
crease of the ethylene selectivity with time on stream was observed
since ethylene was mainly produced by the primary metathesis and side
reaction such as 1-butene self-metathesis and cross-metathesis of pro-
pylene and 1-butene. The butene selectivity of the tungsten catalyst
gradually decreased over time as shown in Fig. 9(D). It is believed that
these tungsten catalysts exhibited high activity for both primary me-
tathesis and secondary metathesis, resulting in the conversion of butene
by secondary metathesis to C5+
.
4. Conclusions
This study demonstrates the effects of the synthesized method and
the low alumina content on the catalytic properties in propene me-
tathesis. The different synthesized methods and 1%Al-doped clearly
play an important role in the changing of surface silanol structure and
acidity. The presence of 1% Al provoked significantly to the catalyst
properties in terms of dispersion, type of acidity, and surface structure
and it directly favored higher metathesis performance. The existence of
1Al2O3-SiO2(SP) > W-imp
1Al2O3-SiO2(SP) > W-imp
1Al2O3/
SiO2(gel) > W-SiO2(SP) > W-imp SiO2(SP) and W-imp SiO2(gel). The
abundance of Bronsted and Lewis acid was found to play important role
in the propene metathesis and the secondary metathesis of 1-butene
reactions. The specific activities per gram-cat and per gram-W of the
catalysts in this study were comparable to those reported in the
Fig. 9. The catalyst performance of tungsten
catalysts in propene self-metathesis, (A) pro-
pene conversion and (B-D) product selectivity:
W-imp SiO2(gel),
W-imp SiO2 (SP),
W-SiO2 (SP), W-imp 1Al2O3/SiO2(gel),
W-imp 1Al2O3-SiO2 (SP), and
SiO2 (SP).
W-1Al2O3-
7

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Table 3
Comparison of specific activity of supported W catalysts from different studies.
Catalyst
Synthesis method
Pretreatment
Reaction conditions
Specific activity1
Ref.
Mol/g cat/ Mol/g W/
h
h
5Wa/SiO2
5Wa/SiO2
Impregnation on silica gel
550 ◦C in air for 1.5 h and 550 ◦C in 500 ◦C, 0.1 MPa2 9.96c
CO for 15 min mmol/s.kg
550 ◦C in air for 1.5 h and 550 ◦C in 500 ◦C, 0.1 MPa2 9.96c
6.78
16.72
13.56
33.44
[53]
[53]
Impregnation on silica with solution of
H3PW12O40.xH2O
Impregnation on spherical support
Incorporated spherical catalyst
Impregnation on zeolite HY (Si/Al
ratio = 500)
CO for 15 min
mmol/s.kg
8Wa/SiO2
8Wa/SiO2
8Wa/HY
500 ◦C in N2 for 1 h
500 ◦C in N2 for 1 h
500 ◦C in N2 for 1 h
550 ◦C, 1 atm 4.4b h−1
550 ◦C, 1 atm 4.4b h−1
550 ◦C, 1 atm 8.7b h−1
5.69
7.08
3.14
7.12
8.85
3.93
[15]
[15]
[4]
8Wa/HY
W-SiO2(SP)
Impregnation on Hierachical HY zeolite
Incorporated spherical catalyst
500 ◦C in N2 for 1 h
500 ◦C in N2 for 1 h
500 ◦C in N2 for 1 h
500 ◦C in N2 for 1 h
550 ◦C, 1 atm 8.7b h−1
550 ◦C, 1 atm 4.08b h−1
550 ◦C, 1 atm 4.08b h−1
550 ◦C, 1 atm 4.08b h−1
4.29
7.69
8.28
8.34
5.36
8.55
9.20
9.26
[4]
This work
This work
This work
W-imp 1Al2O3-SiO2(SP) Impregnation on spherical support
W-1Al2O3-SiO2(SP)
Incorporated spherical catalyst
1
2
Propene converted per grams-catalyst (or per grams-W) per hours, 0.1 MPa ≈ 0.99 atm.
a
b
c
wt.% W loading, WHSV (h−1), Space velocity (mmol/s.kg).
strong acid and inferior surface silanol groups were obtained by in-
corporating spherical W catalysts that directly influenced to higher
metathesis activity. Appropriate surface silanol structure was suggested
to provide the formation of highly active W species for both primary
and secondary metathesis reactions.
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CRediT authorship contribution statement
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Declaration of Competing Interest
The authors declare that there are no conflicts of interest.
Acknowledgements
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Financial supports from the SCG Chemicals Co, Ltd., the Thailand
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