Nature of WOx Sites on SiO2 and Their Molecular Structure-Reactivity/Selectivity Relationships for Propylene Metathesis

Article abstract of DOI:10.1021/acscatal.6b00389

Supported WO_x/SiO₂ catalysts were investigated for propylene metathesis as a function of tungsten oxide loading and temperature. The catalysts were synthesized by incipient-wetness impregnation of an aqueous ammonium metatungstate solution onto the silica support and calcined at elevated temperatures to form the supported tungsten oxide phase. In situ Raman spectroscopy under dehydrated conditions revealed that below 8% WO_x/SiO₂, only surface WO_x sites are present on the silica support: dioxo (O=)₂WO₂ and mono-oxo O=WO₄. The in situ XANES analysis showed that dioxo surface WO₄ sites were the dominant surface WO_x sites on SiO₂ (>90%). The isolated nature of the surface WO_x sites was confirmed with in situ UV-vis spectroscopy. The surface WO_x sites are activated by exposure to propylene at elevated temperature that removes oxygen from these sites. The activation process produces a highly active surface WO_x site that can perform olefin metathesis at 150-250 °C. For 8% WO_x/SiO₂ and higher tungsten oxide loading, crystalline WO₃ nanoparticles (NPs) are also present, and their amount increases with greater tungsten oxide loading. WO₃ NPs, however, are not active for propylene metathesis. The acid character of the surface WO_x sites (Lewis) and WO₃ NPs (Br?nsted) is responsible for formation of undesirable reaction products (C₄-C₆ alkanes and dimerization of C₂⁼ to C₄⁼). This study represents the first time that molecular level structure-activity/selectivity relationships have been established for propylene metathesis by conventionally impregnated supported WO_x/SiO₂ catalysts.

Full text of DOI:10.1021/acscatal.6b00389

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Article
x
2
Nature of WO Sites on SiO and their Molecular Structure-
Reactivity/Selectivity Relationships for Propylene Metathesis
Soe Lwin, Yuanyuan Li, Anatoly I Frenkel, and Israel E. Wachs
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00389 • Publication Date (Web): 04 Apr 2016
Downloaded from http://pubs.acs.org on April 12, 2016
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Nature of WO Sites on SiO and their Molecular Structure-Reactivity/Selectivity
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Relationships for Propylene Metathesis
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a
b
b
a*
Soe Lwin , Yuanyuan Li , Anatoly I. Frenkel , Israel E. Wachs
a
Operando Molecular Spectroscopy and Catalysis Laboratory, Department of Chemical
Engineering, Lehigh University, Bethlehem, PA 18015, USA
b
Department of Physics, Yeshiva University, New York, NY 10016, USA
*
Corresponding author: iew0@lehigh.edu
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Abstract
Supported WO /SiO catalysts were investigated for propylene metathesis as a function
x
2
of tungsten oxide loading and temperature. The catalysts were synthesized by incipientꢀwetness
impregnation of an aqueous ammonium metatungstate solution onto the silica support and
calcined at elevated temperatures to form the supported tungsten oxide phase. In situ Raman
spectroscopy under dehydrated conditions revealed that below 8% WO /SiO , only surface WO
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sites are present on the silica support: dioxo (O=) WO and monoꢀoxo O=WO . The in situ
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XANES analysis showed that dioxo surface WO sites were the dominant surface WO sites on
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SiO (>90%). The isolated nature of the surface WO sites was confirmed with in situ UVꢀvis
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spectroscopy. The surface WO sites are activated by exposure to propylene at elevated
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temperature that removes oxygen from these sites. The activation process produces a highly
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active surface WO site that can perform olefin metathesis at ~150ꢀ250 C. For 8% WO /SiO
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and higher tungsten oxide loading, crystalline WO nanoparticles (NPs) are also present and their
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amount increase with greater tungsten oxide loading but WO NPs, however, are not active for
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propylene metathesis. The acid character of the surface WO sites (Lewis) and WO NPs
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(Brønsted) are responsible for formation of undesirable reaction products (C ꢀC alkanes and
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=
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dimerization of C to C ). This study represents the first time that molecular level structureꢀ
activity/selectivity relationships have been established for propylene metathesis by
conventionally impregnated supported WO /SiO catalysts.
x
2
Keywords: catalyst, supported, tungsten oxide, silica, metathesis, propylene;
Spectroscopy, in situ, operando, Raman, UVꢀvis, XAS, TPSR.
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1
. Introduction
The olefin metathesis reaction was discovered in the early 1960s and became a
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commercial process in the late 1960s for the production of ethylene and butene from propylene
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(Phillips Triolefin Process). Nowadays, the reverse process is economically desired due to a
world shortage of propylene caused by the shift to lighter feedstocks derived from shale gas
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fracking. Although homogeneous metathesis catalysts have received much attention, with wellꢀ
defined model catalysts that exhibit high activity and selectivity, their commercial applications
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remain quite limited because of issues related to catalyst recovery and product separation. To
circumvent these technical issues, the commercial catalyst employed for the Phillips Triolefin
o
Process employs a heterogeneous supported WO /SiO catalyst that operates at 400ꢀ600 C. The
x
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supported WO /SiO catalyst consists of a tungsten oxide phase deposited on a high surface area
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SiO support. The advantages of the supported WO /SiO catalyst are (i) resiliency to trace
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quantities of oxygenate in the feed that are poisons for metathesis , (ii) long catalyst lifetime
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compared to other supported metathesis catalysts based on MoO or ReO , (iii) ease of catalyst
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regeneration to remove deposited coke and (iv) absence of catalyst embrittlement upon periodic
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regeneration. The higher operating temperature of the supported WO /SiO catalysts, however,
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also results in some olefin isomerization and cracking that adversely impact the metathesis
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selectivity.
From early Raman spectroscopy, under ambient conditions where the catalyst is
hydrated, and H ꢀtemperature programmed reduction studies of supported WO /SiO catalysts it
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was proposed that the catalytic active site is a surface compound and not crystalline WO3
nanoparticles (NPs), but the molecular structure of the surface WO compound was not
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determined. Van Roosmalen et al. initially proposed that the active surface WO sites consist of
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pentaꢀcoordinated monoꢀoxo O=W(OH) (ꢀOꢀSi) and O=W(OH)(ꢀOꢀSi) coordinated species,
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but direct evidence for these molecular structures was not provided. Verpoort et al. investigated
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a support consisting of a thin SiO film on a Si(100) single crystal and reacted the silica layer
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with C H W(CO) Cl in a PhCl solvent to prepare the model catalyst to examine the nature of the
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deposited WO layer after evaporation of the solvent at 120 C with angle resolvedꢀXPS in
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ꢀ10
vacuum.
The study revealed that the deposited tungsten oxide phase was present as a highly
dispersed oxide layer after calcination and it was proposed that the surface tungsten oxide was
present as isolated surface WO and polymeric WO sites contained 2 SiꢀOꢀW bonds and
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terminated with either W–OH or W=O functionalities. These model XPS studies, however, can
only provide the average stoichiometry of the surface tungsten oxide sites and not direct
molecular structural information. Basrur et al. proposed, without supporting experimental
evidence, that dioxo (O=) W(ꢀOꢀSi) are the catalytic active sites that become nonstoichiometric
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upon activation for metathesis. Martin et al. performed EXAFS measurements under ambient
conditions on supported WO /SiO catalysts under ambient conditions and concluded that either
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hydrated polytungstate chains or Siꢀcontaining Kegginꢀtype clusters are present on silica.
Several more recent publications about olefin metathesis by supported WO /SiO catalysts have
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been reported, but the characterization studies were also performed under ambient conditions
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20,21
where only hydrated polyoxo (W O ) clusters and WO NPs are known to be present.
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These studies, however, incorrectly assigned the Raman bands of the hydrated polyoxo tungsten
oxide clusters as those belonging to dehydrated surface WO sites because of an earlier incorrect
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assignment by in the literature by Huang et al. The molecular and electronic structures of the
tungsten oxide phases on silica have been determined by Wachs et al. combining in situ Raman
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and UVꢀvis spectroscopy.
Under ambient conditions, hydrated polyoxo (W O ) clusters
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are the dominant surface tungsten oxide species
and WO nanoparticles (NPs) are also
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present with increasing tungsten oxide loading. Under dehydrated conditions, the hydrated
tungsten oxide polyoxo clusters decompose to isolated surface dioxo, (O=) W(ꢀO) , and monoꢀ
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oxo O=W(ꢀO) species and the WO NPs remain unchanged.
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In wellꢀdefined silicaꢀsupported organometallic systems, organic precursors are grafted
onto partially dehydroxylated silica (at 200ꢀ800°C) and these systems are able to perform
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metathesis of both linear and functionalized olefins at relatively low temperatures (<100°C).
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Some of the Wꢀorganic precursors reported in the recent literature are [(≡SiꢀOꢀ)W(=O)Me3],
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[
W(=O)(=CHtBu)ꢀ(SHMT) (PMe Ph)], [W(=O)(=CHtBu)(SHMT) ] (SHMT = 2,6ꢀ
2 2 2
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dimesitylthiophenoxide), (ArO) W(=O)(=CHtBu) (ArO
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=
2,6ꢀmesitylphenoxide), [WOCl ] with SnMe , WONp , Np = CH ꢀtBu, and
4 4 4 2
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[
WO(CH SiMe ) Cl]. These wellꢀdefined systems have been characterized with NMR,
2 3 3
XANES/EXAFS and IR experiments and DFT calculations. Although their catalytic activity is
extremely high, such catalysts cannot be applied in practice to large scale olefin metathesis
systems due to their air sensitive nature and inability to regenerate such catalysts. These model
silicaꢀsupported organometallic catalysts, however, are providing many fundamental insights
about the olefin metathesis reaction.
In the present investigation, systematic in situ and operando UVꢀvis, XAS
(
XANES/EXAFS) and Raman spectroscopy under dehydrated and metathesis reaction conditions
with transient and steadyꢀstate reaction measurements. This is the first study to report on the
nature of the supported tungsten oxide phase on silica during the olefin metathesis reaction that
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allows establishing the molecular structureꢀactivity/selectivity relationships for propylene
metathesis by conventionally impregnated supported WO /SiO catalysts.
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2. Experimental
2.1. Catalyst Synthesis
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The SiO support (Cabot, BET surface area= 332 m /g) was water treated and dried under
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ambient conditions overnight before being calcined in air at 500°C for 4 hours. Incipientꢀwetness
impregnation of WO is achieved by using ammonium metatungstate hydrate, ((NH ) H W O
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·
xH O, Pfaltz and Bauer, 99.5%) in an aqueous solution of water. The samples were initially
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dried at RT overnight, then at 120°C in air for 2 hours before being calcined in air at 500°C for 4
hours.
2.2. In situ UV-vis Spectroscopy
The UVꢀvis spectra of the catalyst samples were taken using a Varian Cary 5E UVꢀvisꢀ
NIR spectrophotometer with the Harrick Praying Mantis accessory. Approximately 5ꢀ25 mg of
each catalyst in finely ground powder form was loaded into an in situ environmental cell
(Harrick, HVCꢀDR2). Spectra of the dehydrated samples were collected in the 200ꢀ800 nm range
o
o
at 25 C, after the 500 C dehydration, using a scan rate of 15 nm/min and a signal averaging time
of 0.6 seconds. A magnesium oxide sample was used as a standard for obtaining the background
absorbance. The KubelkaꢀMunk function F(R ) was calculated from the absorbance of the UVꢀ
∞
vis spectra. The edge energy (E ), or band gap, was determined by finding the intercept of the
g
2
straight line for the lowꢀenergy rise of a plot of [F(R )hν] versus hν, where hν is the incident
∞
3
0
photon energy. An example of this calculation can be found elsewhere. For the in situ UVꢀ vis
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experiments, the supported 8%WO /SiO catalyst was dehydrated at 500°C in 10% O /Ar before
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the temperature was lowered to 300°C under the flow of the same gas and the first spectrum was
taken. The gas flow was then switched to Ar and then to 1% C =/He. The in situ UVꢀvis spectra
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were collected at 300, 350 and 400°C under flowing 1% C =/He. The catalyst was allowed to
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stabilize for 30 minutes at each reaction temperature before the spectrum was recorded. The flow
rate of 30 mL/min is used for all gases in these in situ experiments.
2.3. In situ and operando Raman Spectroscopy
The Raman spectra of silica supported tungsten oxide catalysts were obtained with a
HoribaꢀJobin Ybon LabRam HR instrument equipped with three laser excitations (532, 442 and
325 nm) and a liquid N ꢀcooled CCD detector (HoribaꢀJobin Yvon CCDꢀ3000V). The 442 nm
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laser was chosen since it minimized sample fluorescence. Spectral resolution was approximately
ꢀ
1
ꢀ1
2
cm and the wavenumber calibration was checked using the silica standard line at 520.7 cm .
The lasers were focused on the samples with a confocal microscope using a 50X objective
Olympus BXꢀ30ꢀLWD). Typically, the spectra were collected for 30 s/scan and 5 scans with a
(
200 ꢁm hole. Approximately 5ꢀ25 mg of each catalyst in powder form was loaded into an
environmental cell (Harrick, HVCꢀDR2) with a SiO window and Oꢀring seals which was kept
2
cool by flowing water. The in situ Raman spectra of the dehydrated catalysts were collected at
room temperature after 500°C dehydration in 10% O /Ar for 30 minutes. A flow rate of 30
2
mL/min of 1% propylene/Ar (balance) was used for the operando Raman studies during
propylene metathesis at 300°C. The gaseous effluent was simultaneously recorded with an online
mass spectrometer (Varian, 1200L quadrupole).
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.4. In situ/Operando XAS (XANES/EXAFS) Spectroscopy
W L ꢀedge Xꢀray absorption spectroscopy (XAS) measurements were performed in
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transmission mode at the beamlines X19A and X18B at the National Synchrotron Light Source
NSLS) at the Brookhaven National Laboratory, using ionization chamber detectors for
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measuring incident and transmitted beam intensities. In addition, a third ionization chamber was
used to detect the beam through a reference W foil for energy calibration and alignment
purposes. A quartz capillary cell (I.D./O.D. = 0.8 / 1.0 mm) was used for in situ dehydrated
measurements. Data processing and analysis were performed using Athena and Artemis
software. The in situ XAS spectra of the dehydrated catalysts were collected at the reaction
temperature of 300°C after 500°C dehydration in 20%O /He for 30 minutes. Reference
2
compounds, Ce (WO ) (Strem, 99.9%), H PW O *nH O (Sigma Aldrich, 99.995%) and
2
4 3
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WO (Alfa Aesar, 99.8%) are diluted with Boron Nitride to have a metal loading of 2ꢀ5% and
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measured under ambient conditions. Only tungstophosphoric acid, H PW O *nH O (also
3
12 40
2
diluted with Boron Nitride) was measured under both ambient and dehydrated conditions (150°C
in 20% O /He for 30 minutes). This dehydration was performed to remove adsorbed moisture.
2
For the in situ XAS experiments, after the aforementioned dehydration procedure, the catalysts
=
were cooled down to 300°C in He. Then a flow of 1% C /He was introduced at the rate of 10
3
mL/min and the spectra were recorded as a function of time. The gaseous products were
simultaneously analyzed online with a residual gas analyzer (RGA, Stanford Research System).
2
.5. Temperature programmed surface reaction (TPSR) spectroscopy
The temperature programmed surface reaction experiments were performed using an
Altamira Instruments (AMIꢀ200) system. The outlet gases were connected to an online mass
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spectrometer (Dymaxicon Dycor, (DME200MS) and a TCD detector for analysis. Typically,
~100ꢀ300mg of catalyst was loaded into the Uꢀtube reactor. Blank tests with known
concentrations of olefins were run for the mass spectrometer (MS) calibration before the
experiments. The signals for the mass spectra were also normalized with catalyst weight for
comparison. The following MS m/z values were used for detection of the reactants and products:
propylene (m/z= 42), ethylene (m/z= 27), 2ꢀbutene (m/z=56), acetaldehyde (m/z=43),
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formaldehyde (m/z=30), carbon dioxide (m/z=44), acetone (m/z= 58), O (m/z=32) and water
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(m/z=18). The MS cracking patterns were carefully determined with blank runs using the
calibration gases and used to correct for the background MS signals. Cracking of propylene gives
rise to m/z =43 and the propylene cracking contributions to CH CHO (m/z=43) were subtracted
3
from the final MS signals. Cracking of 2ꢀ butene gives rise to m/z=27, 28, 30 and 41 values.
Unless otherwise noted, the catalysts were dehydrated in 10% O /Ar at 500°C (30 mL/min) for
2
30 minutes and cooled in flowing Ar (30 mL/min) to the reaction temperature. The desired
reactant amount is achieved by diluting the 5% C =/Ar with additional Ar.
3
2.6. Steady-State Reaction Studies
The catalytic activity measurements were performed in a fixedꢀbed catalytic reactor
under differential conditions (propylene conversion <15%). A separate molecular sieve moisture
trap was installed in the inlet propylene gas line to purify the reactants. Both inlet and outlet gas
lines were heated using external electric heaters to ~200°C to prevent condensation of the
reactants and products. The catalysts were pretreated in 10% O /Ar at 500°C for 30 minutes
2
before cooling down in Ar to 300°C. Then a gas mixture of 1% C H /Ar was introduced at the
3
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rate of ~100mL/min. The products were analyzed using an online gas chromatograph (Agilent
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GC 6890) equipped with flame ionization (Agilent Serial #: USC250823H) and thermal
conductivity (Restek Product #: PC3533) detectors. Conversion was normalized with propylene
flow rate and catalyst weight to obtain the propylene metathesis reactivity (mmol/g/hr). The
reactivity is computed from
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
 mL   min   1 L 
Conversion *Concentraion * Flowrate
 *60
 *



mmol 
 =
min
hr
 
1000 mL

 
(1)
reactivity


g * hr


L 
 * LoadingWeight(g)
mol 


22.4

The conversion of propylene is computed from the change of its GC area with and
without the catalyst at each reaction condition. Selectivity of a product (S ) is obtained from
i
dividing its GC area (x ) from the total area of all products, X (excluding the reactant propylene),
i
which can mathematically be expressed as
xi
S = ×100
(2)
i
X
3. Results
.1. In situ UV-vis Spectroscopy
The in situ UVꢀvis Eg values of the dehydrated supported WO /SiO catalysts along with
3
x
2
those of references are shown in Figure 1. The reference Eg values for tungsten oxide isolated
and oligomeric structures are taken from reference 20. Distortion of isolated WO sites has a
4
modest effect and decreases the Eg value from 4.6 to 4.2 eV. Below 8% WO /SiO , the Eg
x
2
values for the dehydrated catalysts are ~4.2ꢀ4.3 eV indicating the presence of isolated WO sites
x
on the silica support. Above 6% WO /SiO , the Eg value monotonically decreases from ~4.2 to
x
2
3.4 eV with increasing tungsten oxide loading. The decreasing Eg values reflect the presence of
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larger WO domains and are related to the presence of crystalline WO nanoparticles (NPs) in
x
3
these catalysts, which will be shown below with the corresponding Raman spectroscopy.
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11
12
13
14
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59
60
4
.8
isolated WOx
4.4
isolated distorted WOx
4
.0
.6
3
3.2
.8
bulk WO3
2
0
2
4
6
8 10 12 14 16 18 20 22
WO (wt%)
x
Figure 1. In situ UVꢀvis Eg values for the dehydrated supported WO /SiO catalysts as a
x
2
function of surface tungsten oxide coverage with those of reference compounds.
The in situ UVꢀvis spectra during the propylene metathesis reaction are presented in
Figure 2a. The curves in flowing O /Ar and Ar are identical and only the latter is evident in
2
Figure 2a since the Ar curve overlaps the O /Ar curve. The difference spectra in Figure 2b were
2
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1
1
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1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
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4
4
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5
5
6
obtained by subtracting the fully oxidized spectrum from the spectra during reaction. Introducing
Ar after the O /Ar pretreatment does not form UVꢀvis dꢀd bands of reduced WO in the 300ꢀ800
2
x
nm region indicating that the WO sites on SiO do not undergo detectable reduction in the inert
x
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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8
9
0
1
2
3
4
5
6
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8
9
0
1
2
3
4
5
6
7
8
9
0
+6
environment. During propylene metathesis, the intensity of the W LMCT band at ~260 nm
decreases and weak dꢀd band(s) from reduced WO sites appear in the ~300ꢀ800 nm region. The
x
+5
UVꢀvis spectra of reduced WO compounds give rise to dꢀd transitions: W in
x
tris(pyrazolyl)boratoꢀoxoꢀtungsten and (Et N)[WO(S C Ph ) ] complexes exhibit bands at 470
4
2
2
2 2
3
1,32
+4
and 720 nm
and W in the WO reference compound exhibits broad bands at 410 and 460
2
nm (see Figure 2b). The weak and broad UVꢀvis bands in the difference spectra during reaction
+
5
+4
suggest that some reduced surface W and W sites are present during propylene metathesis,
but these weak and broad bands are not wellꢀresolved to assign. The absence of strong UVꢀvis dꢀ
d bands for reduced surface WO sites probably reflects the oxidation of the reduced surface
x
+6
WO sites by the surface intermediates back to W (see section below on catalyst activation).
x
+
5
The presence of W sites have been reported for olefin metathesis over supported WO /Al O
3
x
2
33,34
and WO /ZrO
catalysts with EPR spectroscopy. EPR spectroscopy, however, can’t detect
x
2
+4
+3
W sites since they are not paramagnetic. These EPR measurements did not detect W , which is
+
3
paramagnetic, and reflect the absence of W sites during olefin metathesis by supported WO
catalysts.
x
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(
a) in situ UV vis spectra
1
6
2
8
4
0
3
00
300
300
°C O /Ar
2
8WSi
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11
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59
60
°
C Ar
1
°
C C =
3
350
°
C C =
3
400
°
C C =
3
Increasing Temperature in C =
3
2
00
300
400
500
600
700
800
Wavelength (nm)
(
b) in situ difference UV Vis spectra
4
10
+
4
+4
1.5
+5
W
W
W
+5
300°
C C =
W
470
60
3
4
7
20
350
°
C C =
3
1.0
0.5
400
°C C =
3
+
4
WO (W
)
2
3
00
400
500
600
nm
700
800
Wavelength
(
)
Figure 2. In situ UVꢀvis spectra of the supported 8%WO /SiO catalyst (a) during propylene
x
2
metathesis and (b) difference spectra along with that of the bulk WO reference compound.
2
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3
.2. In situ W L -XANES and EXAFS Spectroscopy
1
The in situ WꢀL XANES spectra of several reference materials with known structures
1
and the dehydrated supported WO /SiO catalysts are presented in Figure 3. The intensity of the
x
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
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9
0
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9
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2
3
4
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1
2
3
4
5
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7
8
9
0
XANES preꢀedge peak increases when the absorbing atom (W) is displaced from the inversion
symmetry center, resulting in a strong preꢀedge feature for WO coordinated structures and a
4
3
5,36
weak preꢀedge feature for WO coordinated structures.
The bulk Ce (WO ) reference
2 4 3
6
37
compound contains only isolated WO sites and, consequently, its XANES spectrum has a
4
strong preꢀedge feature. Bulk crystalline WO is composed of WO ꢀcoordinated units with a
3
6
more symmetric environment and, thus, its XANES preꢀedge feature is quite small. Dehydrated
crystalline H PW O contains highly distorted monoꢀoxo WO sites that deviate from the more
3
12 40
6
symmetric environment of WO giving rise to a weak XANES preꢀedge peak. The dehydrated
3
supported 4% WO /SiO catalyst exhibits almost the same preꢀedge intensity as bulk Ce (WO ) .
x
2
2
4 3
Comparison of the intensity of the XANES preꢀedge peaks for the dehydrated supported 4%
WO /SiO catalyst and Ce (WO ) suggests that the catalyst contains ~92% WO ꢀcoordinated
x
2
2
4 3
4
and ~8% WO ꢀcoordinated sites. The intensity of the XANES preꢀedge for the supported 8% and
6
20% WO /SiO catalysts decreases with tungsten oxide loading reflecting the additional presence
x
2
of a WO ꢀcoordinated structure, which is crystalline WO nanoparticles (NPs) (see Raman
6
3
section below). Comparison of the XANES preꢀedge intensity of the supported 8 and 20%
WO /SiO catalysts with that of the supported 4% WO /SiO catalyst suggests that the catalysts
x
2
x
2
with higher tungsten oxide loading contain ~25 and ~60% WO NPs, respectively.
3
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.2
1.0
.8
.6
.4
.2
(
a)
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60
0
Ce (WO )
2
4 3
0
H PW O (dehydrated)
3
12 40
0
WO3
0
0
.0
1.2
(b)
1
.0
.8
.6
0
0
4%
8%
0.4
0.2
20%
0.0
12075 12100 12125 12150 12175 12200 12225 12250
E (eV)
Figure 3. In situ W L edge XANES spectra of (a) reference materials and (b) dehydrated
1
supported WO /SiO catalysts.
x
2
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5
6
The corresponding in situ W L EXAFS spectra provide information about the radial
1
2
distribution of WꢀO bonds. The k ꢀweighted EXAFS spectra of the reference materials and
dehydrated supported WO /SiO catalysts are shown in Figure 4. All spectra exhibit distinct
x
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
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0
1
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9
0
1
2
3
4
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8
9
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1
2
3
4
5
6
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8
9
0
peaks assigned to WꢀO contribution in the range of 1ꢀ2 Å. As shown in Figure 4a, there are
differences in WꢀO peak intensities and positions, reflecting the differences in WꢀO coordination
environments between different reference compounds. The EXAFS spectra of the dehydrated
supported WO /SiO catalysts (Figure 4b) exhibit similar spectral features. The intensity of WꢀO
x
2
peak (at ~ 1.2 Å) is dependent on WO loading, suggesting the coordination number or/and
x
distribution of WꢀO bond changes as a function of WO loading. To obtain quantitative
x
information on WꢀO bonding environment, EXAFS analysis of the reference Ce (WO ) and the
2
4 3
dehydrated supported WO /SiO catalysts were performed. First, we analyzed the Ce (WO )
4 3
x
2
2
compound in order to obtain the amplitude factor (found to be 0.64±0.09), assuming the
tetrahedral coordination of W by O atoms. Then, the amplitude factor was fixed to 0.64 and
applied to all data sets of catalysts. We performed fitting of all catalysts simultaneously and
examined two models: 1) varying the coordination number (N) of WꢀO bonds while requiring
2
that the disorder factor (σ ) of WꢀO bond is the same for all the data sets, and 2) varying both N
2
and σ for all the data. The fitting quality using the two models is comparable, and they have
2
similar reduced Chiꢀsquared and Rꢀfactors. Due to the strong N ꢀ σ correlation, the WꢀO
coordination number could not be obtained reliably from EXAFS analysis, but the best fit values
of the WꢀO bond distances provided important insights. In the both analysis models, the average
WꢀO bond distances are found similar for all WO /SiO catalysts (1.83±0.02 Å for 4%,
x
2
1
.87±0.03 Å for 8% and 1.82±0.02 Å for 20% WO /SiO ) suggesting that all the catalysts are
x 2
22
dominated by a WꢀO short bond that is known to be present for surface WO sites on SiO .
x
2
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1.5
(a)
1.2
0.9
0.6
0.3
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60
Ce (WO )
2
4 3
WO3
H PW O (dehydrated)
3
12 40
01 . 05
(b)
1.2
4%
0.9
8%
0
.6
.3
20%
0
0.0
0
1
2
3
4
5
Å
R( )
2
Figure 4. Magnitudes of Fourierꢀtransformed, notꢀphaseꢀcorrected, k ꢀweighted W L ꢀedge
1
EXAFS spectra in the R space for the (a) reference materials and (b) dehydrated supported
ꢀ1
WO /SiO catalysts. For all the data, the k range for Fourier transformation is 2ꢀ9 Å .
x
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2
2
2
2
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2
2
2
2
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The in situ XANES spectra of the supported 4% and 20% WO /SiO catalysts during
x
2
o
propylene metathesis at 300 C are shown in Figure S1. The supported 4%WO /SiO catalyst is
x
2
free of WO NPs (see UVꢀvis section above and Raman section below) and exhibits almost no
3
0
1
2
3
4
5
6
7
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9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
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8
9
0
1
2
3
4
5
6
7
8
9
0
decrease in the intensity of the XANES preꢀedge peak during propylene metathesis. The
supported 20% WO /SiO catalyst that contains a significant amount of crystalline WO NPs
x
2
3
(
UVꢀvis section above and Raman section below), however, it also does not exhibit an apparent
2
decrease in the intensity of the XANES preꢀedge peak during propylene metathesis. The k
weighted W L ꢀedge EXAFS data of the supported WO /SiO catalysts are presented in Figure
1
x
2
S2 and do not undergo any significant changes. The corresponding operando MS spectra for
both catalysts are shown in Figure S3.
3
.3. In situ/Operando Raman Spectroscopy
The in situ Raman spectra of the dehydrated supported WO /SiO catalysts before olefin
x
2
metathesis are presented in Figure 5. The Raman spectra are normalized with respect to the 495
ꢀ1
cm band of the SiO support. The silica support gives rise to Raman bands at 410ꢀ415, 487,
2
ꢀ1
605, 810 and 970 cm that have been assigned to network bending modes, D1 and D2 defect
modes related to triꢀ and tetraꢀcycloxiloxane rings, SiꢀOꢀSi symmetrical stretching and SiꢀOH
2
2
stretching mode of the surface hydroxyls, respectively. The supported 4% and 6% WO /SiO
x
2
ꢀ1
catalysts exhibit additional Raman bands at 1016, 985 and 350 cm from dioxo v [(O=) WO ]
s
2
2
ꢀ1
stretch, monoꢀoxo v [O=WO ] stretch and their associated bending modes at ~350 cm ,
s
4
2
2
ꢀ1
respectively. The small shoulder at ~968 cm is the v [(O=) WO ] vibrational stretch of the
as
2
2
surface dioxo site. The ratio of the raw Raman bands for the dioxo/monoꢀoxo surface WO sites
x
is ~1.5 and ~2.5 for the supported 2 and 4% WO /SiO catalysts, respectively, and decreases to
x
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~
0.7 for supported 8 and 20% WO /SiO catalysts (see Figures S4a and S4b in SI). Switching the
x
2
gaseous environment from He to O /He has a minimal effect on the dioxo/monoꢀoxo ratio (see
2
ꢀ1
Figure S5). The absence of Raman bands from bending WꢀOꢀW vibrations in the ~200ꢀ300 cm
10
11
12
13
14
15
16
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60
region demonstrates that the surface WO sites are isolated, which is in agreement with the UVꢀ
x
ꢀ1
vis findings above. Crystalline WO NPs (Raman bands at ~265, ~325, ~715 and ~806 cm )
3
3
8
have a Raman crossꢀsection that is significantly stronger than the surface WO sites, and are
x
not present for the supported 4% and 6% WO /SiO catalysts, but are clearly present for
x
2
supported 8% WO /SiO and higher tungsten oxide loadings. The Raman bands of the symmetric
x
2
stretches of the surface dioxo WO and monoꢀoxo WO sites slightly blue shift in the presence of
4
5
the crystalline WO NPs. The Raman spectra reveal that tungsten oxide is 100% dispersed on
3
SiO below 8% WO /SiO for the current set of synthesized catalysts, which is in agreement with
2
3
2
the above UVꢀvis findings.
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96
1023
20%
8
06
996
1016
985
8%
968
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016
6%
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4%
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0%
263
9
00
950
1000
Raman Shift (cm )
1050
1100
-1
3
25
1
016
3
50
985
20%
8%
6%
4%
0%
2
00
400
600
800
1000
1200
-1
Raman Shift (cm )
Figure 5. In situ Raman spectra (442 nm) of the supported WO /SiO catalysts under dehydrated
x
2
o
ꢀ1
conditions (flowing O /Ar at 300 C). Inset shows the 900ꢀ1100 cm region.
2
The operando Raman spectra of the supported 4% and 8% WO /SiO catalysts during
x
2
propylene metathesis are presented in Figures 6 and 7 with the bands normalized by employing
ꢀ1
ꢀ1
the SiO vibrations at 815 cm for the 4% catalyst and 495 cm for the 8% catalyst as internal
2
ꢀ1
ꢀ1
standards. The strong 805 cm band for WO crystals prevents monitoring of the SiO 815 cm
3
2
band as an internal standard for the 8%WO /SiO catalyst. Therefore, the silica Raman band at
x
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ꢀ1
4
95 cm band was used as an internal standard for 8%WO /SiO catalyst. Prior to propylene
x 2
metathesis, the catalyst were always flushed with flowing Ar to remove gas phase molecular O2
=
before beginning the flow of C /Ar. The Raman spectra in flowing 10% O /Ar and Ar were the
3
2
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same in both environments. The in situ Raman spectra of the supported 4% WO /SiO catalyst
x
2
(Figure 6a) indicate that the intensity of the Raman bands of both surface dioxo (O=) WO and
2
2
monoꢀoxo O=WO sites decrease during propylene metathesis and both bands are present for at
4
least ~100 minutes of reaction. The intensity of the Raman bands from both surface WO sites
x
selectively decrease with reaction time relative to the irreducible SiO support. This indicates
2
that both surface WO sites have been activated by loss of W=O oxo bonds formation of surface
x
W=CH /W=CHCH intermediates and that the intensity decrease is not related to deposition of
2
3
carbonaceous deposits. The decrease in W=O vibrations is due to removal of oxygen and
coordination of olefins and their reaction intermediates to the surface WO sites. Within
x
experimental error, it is difficult to determine if one of the surface WO sites reduces faster than
x
the other. The corresponding simultaneous MS signals confirm that the propylene metathesis
=
=
4
reaction is taking place and that the MS ratio of C /C ~1. Both surface WO sites remain
2
x
dispersed on the silica support during propylene metathesis since reoxidation does not reveal
crystalline WO NPs and the relative ratio of the surface dioxo/monoꢀoxo is the same at the
3
beginning of the reaction and after reaction, The behavior of crystalline WO NPs during
3
propylene metathesis is presented in Figure 7 for the supported 8% WO /SiO catalyst. The
x
2
intensity of the Raman bands for the crystalline WO NPs monotonically decreases with
3
propylene metathesis reaction time indicating reduction of these sites. Complete recovery of the
intensity of the Raman bands from the crystalline WO NPs is obtained upon reoxidation
3
indicating that the loss in intensity is related to reduction of the crystalline WO NPs during
3
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metathesis. The corresponding simultaneous MS signals show that propylene metathesis is taking
=
=
place with a ratio of only C /C ~0.1. The almost order of magnitude decrease in formation of
2
4
ethylene must be related to the significant amount of crystalline WO NPs in the support 8%
3
0
1
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9
0
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9
0
WO /SiO catalyst (~25%) since it is the only difference between the type of tungsten oxide sites
x
2
in the supported 8% and 4% WO /SiO catalysts. The formation of some polyaromatic coke on
x
2
ꢀ1
ꢀ1
both catalysts is reflected by Raman bands at 1350 cm and 1600 cm (SI Figure S6).
8
10
(
a) 4WSi
(b) 4WSi
300 C 1%C =/Ar
985
4000
300 C 1%C =/Ar
°
1016
3
°
3
C =
3
7
0
1
m/z=42
0
min
3000
2000
1000
0
4
0 min
0 min
20 min
8
6
C =
2
10
1
m/z=27
C =
4
pure SiO2
5
10
m/z=56
reoxidized
6
00 700 800 900 1000 1100 1200
0
20
40
60
80
)
100 120
-1
Raman Shift (cm )
Time (min
Figure 6. (a) Operando Raman spectra (442 nm) of the supported 4%WO /SiO catalyst (Raman
x
2
o
spectrum of pure SiO is shown for comparison) at 300 C, and (b) its simultaneous MS spectra
2
for main products. The catalyst was reoxidized in 10% O /Ar at 500°C for 30 minutes after the
2
ꢀ1
reaction (Raman spectra normalized against SiO band at 815 cm ). Propylene conversion was
2
~4%.
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ACS Catalysis
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8
2
5000
10
(
a) 8WSi
00 C 1%C =/Ar
(b) 8WSi
300 C 1%C =/Ar
3
°
°
3
3
C =
3
806
7
0
1
20000
m/z=42
712
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1
0
0 min
0 min
0 min
C =
4
1
5000
4
8
m/z=56
5
10
C =
2
10000
1
20 min
m/z=27
4
0
1
5
000
reoxidized
00 700 800 900 1000 1100 1200
3
0
1
6
0
20
40
60
Time min)
80
100 120
-
1
Raman Shift (cm )
(
Figure 7. (a) Operando Raman spectra (442 nm) of the supported 8%WO /SiO catalyst at
x
2
o
3
00 C, and (b) simultaneous MS spectra for the main products. The catalyst was reoxidized in
0% O /Ar at 500°C for 30 minutes after the reaction (Raman spectra normalized against SiO
1
2
2
ꢀ1
ꢀ1
band at 495 cm because of strong WO band at 806 cm ). Propylene conversion was ~9%.
3
o
3.4. Activation of Supported WO /SiO Catalysts at Constant Temperature (500 C)
x
2
=
The supported 8% WO /SiO catalyst was activated by exposure to flowing C /Ar at
x
2
3
5
00°C and the results for the first 60 minutes are presented in Figure 8. The initial product is a
spike of O most probably from displacement of a small amount of residual oxygen from the
2
=
reactor by the flowing C /Ar. Immediately afterwards, the metathesis of propylene to ethylene
3
and butene reaction began and oxygenated products (CH CHO, HCHO, H O and CO ) also
3
2
2
appear. Note that the oxygenated product acetone, CH COCH , was not detected.
3
3
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2
2
.00E-007
.50E-007
.00E-007
(
a) Olefin Products
8
WSi
0
1
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9
0
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500°C
C = m/z=42
3
1.50E-007
1.00E-007
5.00E-008
C = m/z=56
4
C = m/z=27
2
0.00E+000
6.00E-008
5.00E-008
4.00E-008
3.00E-008
2.00E-008
1.00E-008
0.00E+000
(b) Oxygenated Products
H O m/z= 18
2
O m/z= 32
2
CH CHO m/z= 43
3
HCHO m/z= 30
CO m/z= 44
2
C H O m/z= 58
3
6
0
10
20
30
40
50
60
Time (min)
Figure 8. Activation of supported 8% WO /SiO catalyst by flowing 1% C =/Ar at 500°C: (a)
x
2
3
olefin products and (b) oxygenated products. Propylene conversion was ~18%.
3
.5. Activation of Supported WO /SiO Catalysts during TPSR spectroscopy
x 2
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=
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The production of C during activation of supported WO /SiO in flowing C in the
4
x
2
3
=
temperature programmed mode is presented in Figure 9 as a function of WO loadings. The C ꢀ
x
4
TPSR spectra for the supported 4% and 8% WO /SiO catalysts are very similar (same Tp
x
2
10
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value), with the 4% WO /SiO catalyst being slightly more active at the lower temperatures. The
x
2
=
lack of increased production of C in going from the 4% WO /SiO to the 8% WO /SiO
2
4
x
2
x
catalyst reveals that the crystalline WO NPs in the latter don’t play a significant role in olefin
3
=
metathesis and may actually diminish C formation. This is further confirmed for the production
4
=
of C during TPSR with the 20% WO /SiO catalyst, which consists of ~65% WO NPs, that
4
x
2
3
=
=
yields even less formation of C . The leading edges for C formation are almost the same for
4
4
the 8% and 20% WO /SiO catalysts.
x
2
C =
4
2.50E-008
m/z=56
630
°
C
8%
4%
2.00E-008
1.50E-008
=
600°C
5
%C /Ar
3
30mL/min
1
.00E-008
.00E-009
20%
5
0.00E+000
300
400
500
600
700
800
Temperature (°C)
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=
=
Figure 9. Production of C during TPSR in flowing 5% C /Ar as a function of WO loadings.
4
3
x
Other reaction products are shown in Figure S7.
0
1
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9
0
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0
=
The simultaneous evolution of H O and CO during C activation in the TPSR mode is
2
2
3
presented in Figure 10. Formation of these oxidation products confirms the removal of oxygen
atoms from the supported tungsten oxide phases on silica. The characteristics of H O evolution is
2
dependent on the tungsten oxide loading. The supported 4% WO /SiO catalyst, only containing
x
2
o
the isolated surface WO sites, evolves H O in one major peak with Tp=630 C. The higher
x
2
loading catalysts, containing both isolated WO sites and WO NPs, exhibit two H O peaks at
x
3
2
o
o
~
485 and ~630 C. The production of H O at ~630 C is related to isolated surface WO sites on
2
x
silica since this peak is observed for the 4% WO /SiO catalyst that only contains isolated sites.
x
2
o
The second H O peak at ~485 C, however, is associated with the WO NPs and the intensity of
2
3
this peak tracks their concentration in the catalysts. Integration of the H OꢀTPSR peaks indicates
2
o
that the ~485 C peak accounts for ~40% and ~60% of the total H O produced for the 8% and
2
2
0% WO /SiO catalysts. These values reasonably track the amounts of crystalline WO in these
x
2
3
o
catalysts estimated above from XANES analysis. The evolution of CO initiates at ~600 C and
2
o
o
o
peaks at ~680 C. The formation of H O below 600 C and formation of CO above 600 C
2
2
suggests that some carbonaceous deposits are present on the catalyst during activation of the
o
supported WO /SiO catalysts by propylene below 600 C
x
2
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.00E-007
.00E-008
=
H O
5
%C /Ar
3
2
3
0mL/min
m/z=18
8
630°C
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Surface WOx
WO3
485°C
6.00E-008
2
0%
4.00E-008
8%
2
.00E-008
.00E-008
4%
1
20%
CO
8.00E-009
2
m/z=44
6.00E-009
8%
4
.00E-009
.00E-009
4%
2
300
400
500
600
700
Temperature (°C)
=
Figure 10. Production of H O and CO in flowing 5% C /Ar as a function of WO loadings.
2
2
3
x
=
=
The simultaneous conversion of C and formation of C are shown in Figure S7.
3
2
The influence of propylene in the gas phase on the supported 8% WO /SiO catalyst preꢀ
x
2
o
activated with propylene at 500 C for 45 minutes followed by additional exposure to propylene
o
=
at 100 C for 45 minutes is presented in Figure 11. When the TPSR is performed in flowing C ꢀ
3
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=
free Ar, no butene is formed. Performing TPSR in flowing C /Ar after the catalyst activation
3
o
=
o
treatment produces butene at both ~220 and ~600 C. The C peak at ~600 C is also observed
4
during TPSR with an unactivated catalyst (see Figure 9), which suggests that the new low
0
1
2
3
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5
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9
0
1
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1
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9
0
=
=
temperature C peak is related to a new activated surface WO site. The C ꢀTPSR findings also
4
x
3
suggest that surface reaction intermediates are not stabilized on activated WO /SiO catalysts and
x
2
the metathesis reaction requires the presence of propylene in the gas phase in order to proceed.
8.00E-008
6.00E-008
4.00E-008
2.00E-008
600°C
C =
4
m/z=56
8
%WO /SiO
x
2
220°C
5
% C3=
1
% C3=
0%C3=
0.00E+000
100
200
300
400
500
600
700
Temperature (°C)
=
=
3
=
Figure 11. Production of C4 during C ꢀTPSR with 8% WO /SiO catalyst in flowing C /Ar as
x
2
3
=
3
=
a function of propylene concentration after a 500°C C /Ar treatment for 45 minutes and C .
3
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o
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adsorption at 100 C before starting the C ꢀTPSR. Other reaction products are shown in Figure
3
S8.
3.6. Steady-State Olefin Metathesis Reaction Studies
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The reaction rates (propylene molecules converted per gꢀcat per hour) and selectivity for
propylene metathesis by the supported WO /SiO catalysts as a function of tungsten oxide
x
2
loading are presented in Figures 12 and 13, respectively. The propylene metathesis reaction rate
increases with tungsten oxide loading up to 8% WO /SiO and then levels off for higher tungsten
3
2
oxide loadings. Below 8% WO /SiO , only surface WO sites are present on the silica support
3
2
x
and demonstrate that the reaction rate is proportional to the surface WO sites concentration (the
x
catalytic active sites). Above 8% WO /SiO , the propylene metathesis reaction rate is relatively
3
2
constant reflecting the lack of influence of the WO NPs upon reaction rate (not the catalytic
3
=
=
active sites). The propylene metathesis C /C selectivity is sensitive to the nature of the
2
4
tungsten oxide sites present in the catalyst. Below 8% WO /SiO , where only surface WO sites
x
2
x
=
=
4
are present, the ratio of C /C ~1 and ~20% C ꢀC alkane byproducts are also formed from the
2
4
6
39
weak Lewis acidy associated with the surface WO sites on SiO . The selectivity to C and C
5
x
2
4
is relatively constant and the selectivity to C increases with tungsten oxide loading from 2ꢀ6%
6
WO /SiO . For 8% WO /SiO and higher loadings, where crystalline WO NPs are present in
x
2
x
2
3
=
addition to the surface WO sites, the selectivity to C dramatically decreases while the
x
2
=
selectivity of C modestly increases with tungsten oxide loading. The selectivity towards
4
alkanes also further increases (C > C > C ) with tungsten oxide loading with increasing amount
6
5
4
of crystalline WO NPs. These selectivity changes are thought to arise from the surface Brønsted
3
4
0,41
acid sites associated with the crystalline WO NPs.
3
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