Enhanced Olefin Metathesis Performance of Tungsten and Niobium Incorporated Bimetallic Silicates: Evidence of Synergistic Effects

Article abstract of DOI:10.1002/cctc.201902131

Tungsten and niobium incorporated KIT-6 (WNb-KIT-6) materials were synthesized by a one-pot sol-gel method. During 2-butene+ethylene metathesis with these materials at 450 °C, the propene yield is maximized (～70 %) at a certain metal composition (～20 wt% W and ～1 wt% Nb), significantly surpassing those reported with an industrial WO₃/SiO₂ catalyst. The enhanced propene yield is attributed to better dispersion of W species on the catalyst surface and the formation of new active site precursors (O=)₂W(O?Si)(O?Nb), as confirmed by XRD, TEM, H₂-TPR, XPS, EXAFS, and ToF-SIMS techniques. Complementary DFT calculations reveal a positive correlation between the O=W=O bond angle and the propene yield. We therefore speculate that the ?O?Nb moiety tunes the electronic environment around the W atom and increases the propene yield.

Full text of DOI:10.1002/cctc.201902131

Accepted Article
Title: Enhanced Olefin Metathesis Performance of Tungsten and
Niobium Incorporated Bimetallic Silicates: Evidence of Synergistic
Effects
Authors: Jian-Feng Wu, Anand Ramanathan, Reinhard Kersting, Amy
Marie Jystad, Hongda Zhu, Yongfeng Hu, Craig P Marshall,
Marco Caricato, and Bala Subramaniam
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10.1002/cctc.201902131
ChemCatChem
RESEARCH ARTICLE
Enhanced Olefin Metathesis Performance of Tungsten and
Niobium Incorporated Bimetallic Silicates: Evidence of
Synergistic Effects
Jian-Feng Wu*[a],[b], Anand Ramanathan[b], Reinhard Kersting[d], Amy Marie Jystad[e], Hongda Zhu[b],[c]
Yongfeng Hu[f], Craig P. Marshall[e],[g], Marco Caricato[e], and Bala Subramaniam*[b],[c]
,
Abstract: Tungsten and niobium incorporated KIT-6 (WNb-KIT-6)
materials were synthesized by a one-pot sol-gel method. During 2-
butene + ethylene metathesis with these materials at 450°C, the
propene yield is maximized (~70%) at a certain metal composition
(~20 wt% W and ~1 wt% Nb), significantly surpassing those reported
with an industrial WO3/SiO2 catalyst. The enhanced propene yield is
attributed to better dispersion of W species on the catalyst surface
and the formation of new active site precursors (O=)2W(O-Si)(O-Nb),
as confirmed by XRD, TEM, H2-TPR, XPS, EXAFS, and ToF-SIMS
widely used in industry to produce propene from ethylene and
butenes. Recently, several groups (Wachs[8,25], Stair[11], Bell[12]
Subramaniam[26]
have reported on the structure-activity
relationship of supported W catalysts. These studies suggest that
W-O-Si type species are the active site precursors for metathesis
and that maximizing such species via catalyst synthesis
techniques is a key to increasing propene yield.
A recent patent[27] reported that the addition of 0.01-10 wt% of
niobium or its oxide into the WO3/SiO2 catalyst (0.1-20 wt% W
loading) promotes the disproportionation of butene to produce
propene. Verpoort et al.[28] reported that anchoring aryloxy
tungsten complexes on NbOx/SiO2 yields higher 2-pentene
metathesis activity than catalysts without NbOx. However, a
detailed understanding of the active sites in these supported
multi-metallic oxides is still lacking. In organometallic chemistry,
ligands are used to tune the electronic and/or steric influence on
metal centers to enhance the yield towards targeted products.[29-
31] A similar analogy may be invoked to explain the functioning of
,
)
techniques. Complementary DFT calculations reveal
a positive
correlation between the O=W=O bond angle and the propene yield.
We therefore speculate that the -O-Nb moiety tunes the electronic
environment around the W atom and increases the propene yield.
Introduction
Metathesis involving light alkenes is gaining importance as an
atom economical route to augment the supply of certain olefinic
feedstocks based on demand.[1] Catalysts investigated widely for
olefin metathesis include WO3, MoO3, and Re2O7 supported on
either SiO2 or Al2O3.[2-24] Because of its robustness towards
poisons and relatively low cost, the WO3/SiO2 catalyst has been
a
WO3/SiO2 catalyst in which the active site precursor
[8,12]
(O=)2W(O-)2
is grafted on the SiO2 framework (Scheme 1a).
In such a scenario, the SiO2 framework alters the electronic
nature of the W center to render it active for metathesis. The
active site precursor (O=)2W(O-)2 possesses two (O-) moieties
that provide the possibility for tuning the electronic nature of the
W center by replacing one Si atom with another element (such as
Nb, Scheme 1b). The sol-gel method[7,15,32,33] allows the synthesis
of such types of structures wherein the Si and Nb sources are first
hydrolyzed to form the framework, following which the W species
are grafted onto the Nb2O5-SiO2 framework.
Motivated by this hypothesis, we synthesized a series of
WNb-KIT-6 mesoporous catalysts with different W and Nb
loadings by sol-gel method. We find that the catalyst performance
is indeed enhanced at certain bimetal loadings, with the W10Nb1-
KIT-6 catalyst (~20 wt% W loading and ~1 wt% Nb loading)
yielding up to ~70% propene yield at steady state. This
substantially exceeds the performances of W-KIT-6[15] and W-
EISA[17] catalysts (~60% propene yield) at identical reaction
conditions. The enhanced performance is attributed to the
traditional W dioxo species and the formation of new active site
precursors (O=)2W(O-Si)(O-Nb), which were confirmed by H2-
TPR, XPS, EXAFS and ToF-SIMS techniques. Our results
suggest that such synergy imparted by the second metal can be
systematically exploited in heterogeneous catalysis.
[a]
[b]
[c]
Prof. Dr. Jian-Feng Wu
State Key Laboratory of Applied Organic Chemistry, Key Laboratory
of Nonferrous Metals Chemistry and Resources Utilization of Gansu
Province, College of Chemistry and Chemical Engineering, Lanzhou
University, Lanzhou 730000, People’s Republic of China
E-mail: wjf@lzu.edu.cn
Prof. Dr. Jian-Feng Wu, Dr. Anand Ramanathan, Dr. Hongda Zhu,
Prof. Dr. Bala Subramaniam
Center for Environmentally Beneficial Catalysis, The University of
Kansas, Lawrence, KS 66047, United States
E-mail: bsubramaniam@ku.edu; tel, 785-864-2903; fax, 785-864-
6051
Dr. Hongda Zhu, Prof. Dr. Bala Subramaniam
Department of Chemical and Petroleum Engineering, The University
of Kansas, Lawrence, KS 66045, United States
Dr. Reinhard Kersting
Tascon GmbH, Mendelstr. 17, 48149 Münster, Germany
Amy Marie Jystad, Prof. Dr. Craig P. Marshall, Prof. Dr. Marco
Caricato
[d]
[e]
Department of Chemistry, University of Kansas, Lawrence, Kansas
66045, United States
[f]
Prof. Dr. Yongfeng Hu
Canadian Light Source Inc., University of Saskatchewan,
Saskatoon, Saskatchewan S7N 2V3, Canada
Prof. Dr. Craig P. Marshall
[g]
Department of Geology, The University of Kansas, Lawrence, KS
66045, United States
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cctc.201902131
ChemCatChem
RESEARCH ARTICLE
Scheme 1. (a) 2-Butene and ethylene react on WO3/SiO2 catalysts (W-KIT-6
and W-EISA) to generate propene. The W surface active precursor is bonded
to the SiO2 framework. (b) 2-Butene and ethylene react on WNb-KIT-6 catalyst
to form propene. The W surface active precursor is bonded to Si and Nb atoms.
The framework structures (-O-Si and -O-Nb) influence the chemical
environment around the W center.
Figure 1. Wide angle XRD patterns of W10-KIT-6 (Na), W10-KIT-6, Nb1-KIT-6,
and W10Nbx-KIT-6 catalysts (x = 0.25 – 10). The broad band between 15° and
30° is due to amorphous SiO2.
Results
N2 sorption isotherms and the corresponding pore size
distributions of WNb-KIT-6, W10-KIT-6, and Nb1-KIT-6 catalysts
reveal that these catalysts exhibit type IV isotherm and H1
hysteresis loop (Figure S3 and Table S1). The BET surface area,
pore volume, and average pore size of the WNb-KIT-6 catalysts
are in the range of 545–448 m2/g, 0.59–0.53 cm3/g, and ~9.6 nm,
respectively (see Table S1). When the Nb element was
incorporated into the framework of W10-KIT-6 catalyst (SBET = 450
m2/g), the BET surface area increased gradually reaching 545
m2/g in W10Nb10-KIT-6 catalyst. The values of the (Si/W) and
(Si/Nb) ratios in the synthesis gel are close to those measured by
ICP-OES, indicating that the W and Nb species were fully
incorporated into the catalyst (Table S1).
Synthesis and Characterization of the WNb-KIT-6, Nb-KIT-6,
and W-KIT-6 catalysts. Figure 1 shows the wide-angle XRD
patterns of WNb-KIT-6, Nb-KIT-6, and W-KIT-6 catalysts. The
W10-KIT-6 (Na) catalyst (synthesized from Na2WO4, labeled with
“Na” in the parentheses) displays a strong signal corresponding
to crystalline WO3 species, while the W10-KIT-6 catalyst
(synthesized from ammonium metatungstate, represented
without any label) shows a weak signal of crystalline WO3 species,
indicating better W dispersion on the surface. This may be due to
the Na2WO4 reagent possessing a higher hydrolysis rate than
ammonium metatungstate in the presence of HCl. The Nb1-KIT-6
exhibits only a broad band of amorphous SiO2 between 15° and
30°, indicating the Nb species are well dispersed in the framework,
which is also confirmed by TEM mapping results (Figure S6). The
incorporation of Nb in W10Nbx-KIT-6 yields a new peak at
approximately 22.8° which gradually increases between 1 – 10
Nb/Si mol% but not seen at 0.25 and 0.5 Nb/Si mol% (Figure 1).
This new peak is attributed to NbxWyOz species (Figure S1).
Interestingly, no crystalline WO3 reflections were observed when
Nb was incorporated into the SiO2 framework of KIT-6 during the
synthesis, indicating that Nb increases the dispersion of surface
WOx species. Small angle XRD results (Figure S2) show a peak
centered around 0.89-0.91° for W10Nb0.25-2.5-KIT-6 catalysts,
indicating these WNb-KIT-6 catalysts have ordered structure.
However, the W10Nb10-KIT-6 and W10Nb5-KIT-6 catalysts show
either poorly resolved or no peak below 1°, indicating that these
two catalysts have low ordering in their structures. This conclusion
is also confirmed by TEM images (Figures S4a, S4b).
Representative transmission electron microscopy (TEM)
image, scanning transmission electron microscopy-high angle
annular dark-field (STEM-HAADF) image and energy dispersive
X-ray spectroscopy (EDX) mapping of W10Nb1-KIT-6 catalyst are
shown in Figure 2. The TEM images of all WNb-KIT-6 catalysts
are shown in Figure S4. Highly ordered pores and channels are
clearly visible in the TEM image (Figure 2a) and STEM-HAADF
image (Figure 2c), which are consistent with the results from N2
physisorption study (Figure S3). The presence of less resolved
diffraction spots (Figure 2a insert) indicates that the W10Nb1-KIT-
6
catalyst may contain nanocrystals (nano-WO3, as also
confirmed by UV/Vis experiment, vide infra). The nanocrystals (~1
nm) are more clearly seen in the high-resolution STEM-HAADF
image (Figure 2b). The EDX mapping results (Figure 2d) show a
uniform distribution of W, Nb, Si, and O species throughout the
W10Nb1-KIT-6 catalyst. Comparative TEM results of W10-KIT-6
and Nb1-KIT-6 catalysts (Figures S5 and S6) show that the size
of nano-WO3 on W10-KIT-6 is slightly larger (1-2.5 nm, as inferred
from Figure S5b) lending further credence to the hypothesis that
Nb incorporation enhances the dispersion of W species.
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10.1002/cctc.201902131
ChemCatChem
RESEARCH ARTICLE
Metathesis of Ethylene and 2-Butene to Make Propene. The
metathesis activity of Nb1-KIT-6, W10-KIT-6 (Na), W10-KIT-6, and
W10Nbx-KIT-6 catalysts (x = 0.25 – 10) were compared at the
following conditions: T = 450°C, P = 1 atm, WHSV (ethylene and
2-butene) = 2.0 h-1, n(ethylene)/n(2-butene) = 3/1 (Figures 4, S8
and S9). Compared with the W10-KIT-6 (Na) catalyst,[15] the W10-
KIT-6 catalyst (prepared with ammonium metatungstate as
source) showed a remarkable increase in 2-butene conversion
from 46.6 ± 0.7% to 71.7 ± 0.6% and propene yield from 36.6 ±
0.5% to 60.7 ± 0.7%. This enhancement is consistent with better
W dispersion based on wide-angle XRD results (Figure 1).
To further understand the influence of Nb incorporation on the
W-KIT-6 catalysts, the metathesis activities of the W10Nbx-KIT-6
catalysts listed in Figure 3 were measured in continuous runs.
Figure S9 provides the temporal profiles for 2-butene conversion,
propene selectivity and propene yield for the various catalysts.
The equilibrium propene yield at T = 450°C, P = 1 atm,
n(ethylene)/n(2-butene) = 3/1 is ~83 %, which was estimated with
Aspen Plus® 8.6 software (Figure S10). As shown in Figure 4, the
W10Nb1-KIT-6 catalyst provides the maximum values of 2-butene
conversion (79.1 ± 0.6%), propene selectivity (87.9 ± 0.2%), and
propene yield (69.6 ± 0.6%). Temporal 2-butene conversion as
well as propene, 1-butene, and pentenes selectivity profiles for
W10Nb1-KIT-6 are shown in Figure S11. At higher Nb loadings,
these values decrease and are either similar to or less than those
observed with the W10-KIT-6 catalyst. For example, the
performance of W10Nb10-KIT-6 was significantly worse when
compared to the W10-KIT-6 catalyst, which is attributed to the
formation of inactive NbxWyOz species (Figure S1). In contrast,
the performances of W10Nb0.25-KIT-6 and W10-KIT-6 are similar.
This suggests that W and Nb exhibit synergy only at certain
relative loadings.
Figure 2. TEM images of the W10Nb1-KIT-6 catalyst. TEM image (a) and STEM-
HAADF image (c) show the highly ordered pores and channels of the W10Nb1-
KIT-6 catalyst. The insert in the TEM image (a) shows the less resolved
diffraction spots, indicating the W10Nb1-KIT-6 catalyst contains some
nanocrystals. The high-resolution STEM-HAADF image (b) shows the presence
of nanocrystals. The orange box in Figure 2c is chosen for EDX mapping. W,
Nb, Si, and O distributions (d-g) are shown in green, cyan, yellow, and orange,
respectively.
The nature and the coordination of W species in the W-KIT-6
and WNb-KIT-6 catalysts were studied by DR UV/Vis
spectroscopy in the range of 190-800 nm under ambient
conditions (Figure 3). The absorption band at 233 nm is due to
the hydrated isolated tetrahedral [WO4]2- species.[32,34,35] The band
around 270 nm is attributed to hydrated octahedral polytungstate
species.[32,34,35] A well-resolved band around 400 nm is observed
on W10-KIT-6 catalyst, which is due to the crystalline WO3
species.[32,34,35] However, the WO3 species (400 nm) disappeared
in the case of W10Nb2.5-10-KIT-6 materials. At lower Nb loadings,
broad and less-resolved bands around 400 nm were observed in
W10Nb0.25-1-KIT-6 catalysts similar to the W10-KIT-6 catalyst.
These results further strengthen the explanation that Nb
incorporation into the catalyst improves the dispersion of W
species on the catalyst surface, which is in agreement with wide-
angle XRD (Figure 1) and TEM (Figures 2 and S5) results. A
weakly resolved band around 190 nm, in the background of
hydrated isolated tetrahedral [WO4]2- species, is attributed to the
hydrated tetrahedral NbO4 units,[36-38] This band (~190 nm) is
more clearly observed in the Nb1-KIT-6 catalyst sample (Figure
S7).
We hypothesise that the bimetallic silicate possesses a
combination of the two types of sites shown in Scheme 1b. The
enhanced metathesis activity results from both the conventional
W dioxo species anchored to only Si (Scheme 1a) and the more
active W sites anchored to adjacent Si and Nb (Scheme 1b, M =
Nb). For a given W loading, there exists an optimum Nb loading
that maximizes the number of adjacent Nb and Si sites. Higher
Nb loading results in a progressively increasing population of
NbxWyOz, isolated Nb, and polymeric Nb species that are less
active than either of the two active species shown in Scheme 1.
Consequently, the optimum Nb loading that maximizes the
population of the active sites (and the propene yield) is relatively
low compared to the W loading. The propene yield of 69.6 ± 0.6%,
observed with W10Nb1-KIT-6, exceeds those reported with other
W-incorporated mesoporous silicates [59.2 ± 0.5% for W-KIT-6
(Na)[15] and 60.1 ± 1.4% for W-EISA[17]] at the same reaction
conditions. The apparent activities of various catalysts from our
work and those reported by others are compared in Table 1.
Clearly, the W10Nb1-KIT-6 catalyst shows the best apparent
activity value of 9.92 mmol g-1h-1. Interestingly, as seen from
Figure 4, the propene yield and the total acidity follow an identical
trend, as also reported in previous studies.[15,17,39]
Figure 3. DR UV/Vis spectra of fresh W10-KIT-6 and W10Nbx-KIT-6 catalysts (x
= 0.25 – 10). The UV/Vis measurements were performed at ambient conditions.
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10.1002/cctc.201902131
ChemCatChem
RESEARCH ARTICLE
800°C was observed on W10Nb10-KIT-6 catalyst and this peak
shifted to lower temperatures (700-718°C) with decreasing Nb/Si
mol%, indicating the existence of W-Nb interaction. The reduction
peak at 718°C observed with the W10Nb1-KIT-6 catalyst was
assigned to the W-O-Nb structure based on the observation of a
reduction peak with a WO3/Nb2O5 model catalyst at 714°C. The
peak around 818°C on WO3/Nb2O5 catalyst is attributed to the
reduction of Nb2O5 interacted with W species, based on the
reduction peak of Nb2O5 at 860°C (Figure S14).
Figure 4. 2-Butene conversion, propene selectivity, propene yield and total
acidity of Nb1-KIT-6, W10-KIT-6 (Na), W10-KIT-6, and W10Nbx-KIT-6 catalysts (x
= 0.25 – 10) at T = 450°C, P = 1 atm, WHSV (ethylene and 2-butene) = 2.0 h-1,
n(ethylene)/n(2-butene) = 3/1. The propene yields represent the mean of the
steady state values between 5-7 h. The “KIT-6” are omitted in the caption for
brevity.
Table 1. The compared apparent activity for some W-based
catalysts.
Propen
W loading
(wt%)
Apparent activity
(mmol g-1h-1)
Catalyst
e
yield
(%)
W10-KIT-6a
19.2
16.9
19.1
20.0
20.9
20.0
19.2
60.7
31.6
45.6
62.6
69.6
61.7
8.66
4.50
6.50
8.93
9.92
8.79
W10Nb10-KIT-6a
W10Nb5-KIT-6a
W10Nb2.5-KIT-6a
W10Nb1-KIT-6a
W10Nb0.5-KIT-6a
W10Nb0.25-KIT-6a
Figure 5. H2-TPR profiles of (a) Nb1-KIT-6, (b) W10-KIT-6, (c) W10Nb10-KIT-6,
(d) W10Nb5-KIT-6, (e) W10Nb2.5-KIT-6, (f) W10Nb1-KIT-6, (g) W10Nb0.5-KIT-6, (h)
W10Nb0.25-KIT-6, and (i) WO3/Nb2O5 catalysts.
59.6
59.2
60.1
79.7
N.A.
8.50
8.44
8.57
3.03
~2
W-KIT-6 (2 h, 9.2)[15],a 9.2
To further understand the enhanced metathesis activity on the
W10Nb1-KIT-6 catalyst, W 4f XPS studies were performed on
W10Nb1-KIT-6 and W10-KIT-6 catalysts (Figure 6). Compared to
W10-KIT-6, the W10Nb1-KIT-6 catalyst exhibited a higher peak
intensity at approximately 33.9 eV. This result suggests that Nb
incorporation into W10-KIT-6 catalyst increases the population of
the active site precursors. Quantitative estimates of surface
enrichment by W species from XPS data are shown in Table S2.
W-EISA (22.2)[17],a
22.2
[34],b
WO3/SiO2
24.0
9.8
WOx-imp-Si[40],c
a 1 g of catalyst, T = 450°C, P = 1 atm, WHSV (ethylene and 2-
butene) = 2.0 h-1, n(ethylene)/n(2-butene) in feed = 3/1;
b 3 g of catalyst, T = 200°C, P = 3 atm, WHSV (ethylene and 2-
butene) = 1.6 h-1, n(ethylene)/n(2-butene) in feed = 3/1;
c 0.1 g of catalyst, T = 250°C, 8 mL/min reaction feed consisting
of ethylene and trans-2-butene (1⁚1 molar ratio) together with 10
vol.%N2.
Relationship between Metathesis Activity and the Structure
of the Active Sites. To elucidate the nature of the active sites,
the H2-TPR experiments were performed on Nb1-KIT-6, W10-KIT-
6, W10Nbx-KIT-6, and WO3/Nb2O5 samples (x = 0.25 – 10, Figure
5). The Nb1-KIT-6 catalyst showed a silent signal from 100 to
1050°C, indicating strong interaction of the Nb species with the
framework. In contrast, the W10-KIT-6 catalyst exhibited three
reduction peaks at 800, 868, and 928°C, which may be attributed
to the stepwise reduction from WO3 to W(0) [WO3(VI) → W20O58
(V, VI) → WO2(IV) → W(0)][41]. Upon the incorporation of Nb
element into the W10-KIT-6 catalyst, a new reduction peak at
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10.1002/cctc.201902131
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RESEARCH ARTICLE
Figure 6. Photoemission W 4f feature of W10Nb1-KIT-6 and W10-KIT-6 catalysts.
The XAFS technique is a powerful tool that can provide
information on the local symmetry, coordination and oxidation
state of an atom of interest in the catalyst.[42] The Nb L3-edge
XANES spectra for Nb2O5, W10Nb1-KIT-6 and Nb1-KIT-6 samples
under total electron yield (TEY) and fluorescence yield (FY)
modes are shown in Figure 7. Double-featured peaks (2372.95
eV at peak A and 2375.80 eV at peak B) are dominant in Figure
7, accompanied by a weak signal at approximately 2387.05 eV
(peak C). The peaks at A and B are due to the 2p3/2→4d5/2
transition, while the weak signal at C is attributed to the 2p3/2→5s
transition.[42-44] The observed double-featured peaks at A and B
originate from the ligand field splitting of d-orbitals of Nb
atom.[44,45] For the W10Nb1-KIT-6 and Nb1-KIT-6 samples, the
intensity of peak A is greater than peak B, which indicates that the
Nb atom is in octahedral coordination.[42, 45] At the same time, the
W10Nb1-KIT-6 and Nb1-KIT-6 samples show the same absorption-
edge as Nb2O5 (+5) reference, which demonstrates that the
oxidation state of the Nb in W10Nb1-KIT-6 and Nb1-KIT-6 samples
is +5.[42] Besides, we noticed that the total peak area (A and B) of
W10Nb1-KIT-6 catalyst is higher than that of Nb1-KIT-6 when the
W and Nb elements co-exist in the KIT-6 matrix, implying the
depletion of Nb 4d electron density in the W10Nb1-KIT-6
sample.[42] This points to an interaction between W and Nb
elements.
Figure 8. Normalized tungsten L3-edge XANES spectra for W10-KIT-6, W10Nb1-
KIT-6 catalysts, and the reference WO3 at ambient condition.
To further confirm the proposed hypothesis, molecular
specificity and surface sensitivity of time-of-flight secondary ion
mass spectrometry (ToF-SIMS)[6,48-52] was deployed to determine
the surface composition. In the negative ToF-SIMS spectra
(Figure 9), two interesting clusters were detected for W10-KIT-6
-
and W10Nb1-KIT-6 catalysts: the SiWO6 (m/z
= 308) ion
demonstrates the presence of W-O-Si bonds on both catalysts,
while the NbWO6- (m/z = 373) ion confirms the presence of W-O-
Nb bond on W10Nb1-KIT-6 catalyst. Similar findings were also
reported to confirm the formation of Ti-O-V bond on V2O5/TiO2
catalyst[53] and W-O-V bond on V2O5-WO3/TiO2 catalyst[54]. Even
though the population of the species involving W-O-Nb bonds
(Scheme 1b) on the W10Nb1-KIT-6 is low, we hypothesize that
they are significantly more active than the conventional active site
without Nb (Scheme 1a), resulting in a net enhancement of the
metathesis activity.
Figure 7. Nb L3-edge XANES spectra for Nb2O5, W10Nb1-KIT-6 and Nb1-KIT-6
samples at ambient condition. The Nb2O5 sample was measured with total
electron yield (TEY) mode, while the W10Nb1-KIT-6 and Nb1-KIT-6 samples were
measured with fluorescence yield (FY) mode.
The normalized tungsten L3-edge XANES spectra for W10-KIT-
6, W10Nb1-KIT-6 catalysts, and the reference WO3 at ambient
condition are shown in Figure 8. The W10-KIT-6 and W10Nb1-KIT-
6 catalysts show the same absorption edge as the reference
sample WO3, indicating that the W element in calcined W10-KIT-6
and W10Nb1-KIT-6 catalysts are in the oxidation state of +6.[46]
Besides, the W10Nb1-KIT-6 catalyst has greater peak intensity
than W10-KIT-6 catalyst, implying decreased W 5d electronic
density in the W10Nb1-KIT-6 catalyst, which is consistent with Nb
L-edge XANES. This further corroborates possible interaction
between W and Nb elements in the W10Nb1-KIT-6 catalyst.[47] This
conclusion is in agreement with the Nb L3-edge XANES results
(Figure 8).
Figure 9. Partial negative ToF-SIMS spectra of W10-KIT-6 and W10Nb1-KIT-6
catalysts in the mass range m/z = 300-420.
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is shown by the tube-frame region and the frozen silica by the wire-frame. O, Si,
H, W, and Nb is labeled as red, gray, white, blue, and purple colors, respectively.
Discussion
A new possible active site precursor on W10Nb1-KIT-6 is proposed
in Scheme 1b (M = Nb) based on the evidence from H2-TPR, XPS,
EXAFS, and ToF-SIMS experiments. The dominant active site
precursor on the WO3/SiO2 catalyst is (O=)2W(O-Si)2 species, as
shown in Scheme 1a,[8] created by grafting a W precursor on the
SiO2 framework. As shown in Scheme 1b, the new proposed
active site precursor on W10Nb1-KIT-6 is (O=)2W(O-Si)(O-Nb), in
which one Si atom is replaced by an Nb atom, when a Nb
precursor is also grafted on the SiO2 framework. The
electronegativity (Pauling scale) of Nb atom is 1.60, while that of
the Si atom is 1.90.[55] The -O-Nb and -O-Si structures coordinate
to the tungsten atoms and tune their activity. This hypothesis was
confirmed by the Nb L3-edge XANES and W L3-edge XANES
results (Figures 7 and 8). Similar electronic effects have been
reported in the case of homogeneous Ru-based olefin-metathesis
catalysis based on ligand design.[56,57]
To further understand the relationship between structure and
catalytic activity, we synthesized and evaluated a series of W10M1-
KIT-6 (M = Zr, Hf, and Ta) catalysts (Figure S15 and Table S2).
Since all the W atoms are coordinated by two different OM groups
(M = Si, Zr, Hf, Ta, and Nb), the question arises whether the
O=W=O bond angles in the bimetallic catalysts may influence the
metathesis activity similar to how the dihedral angle dictates the
performance of Ru-P based catalysts in homogenous catalysis.[57]
To compute the O=W=O bond angles, ten models for each of the
bimetallic W10M1-KIT-6 (M = Zr, Hf, Ta, and Nb) catalyst were
considered. One of the proposed active site precursor for W10Nb1-
KIT-6 used in the computation is shown in Figure 10, while those
for the other bimetallic catalysts are shown in Figure S16.
Interestingly, a linear correlation is apparent for the calculated
O=W=O bond angle and the initial propene yield on W10-KIT-6
and W10M1-KIT-6 (M = Zr, Hf, Ta, and Nb) catalysts (Figure 11
and Scheme 2). Experimental evidence of active site precursors
similar to those on W10Nb1-KIT-6 catalyst are shown in Figure
S17. The foregoing results suggest that if the dopant and W form
active site precursors as shown in Scheme 2, the O=W=O bond
angle in such W-based bimetallic catalyst can be used to predict
the relative metathesis activity of the doped catalysts. Given that
the metathesis activity follows the same trend as acidity,[15,17,39]
establishing a definitive link between bond angle and acidity in
future investigations will shed further fundamental insights into the
underlying mechanism and guide the rational design of
metathesis catalysts.
Figure 11. Enhancement of metathesis activity with increasing O=W=O bond
angle for W10-KIT-6 and W10M1-KIT-6 (M = Zr, Hf, Ta, and Nb) catalysts.
Scheme 2. Apparent correlation between O=W=O bond angle and propene
yield in W10-KIT-6 and W10M1-KIT-6 (M = Zr, Hf, Ta, and Nb) catalysts.
Conclusion
We demonstrate a one-pot synthesis technique to prepare W-
KIT-6 catalysts doped with Nb (WNb-KIT-6). At a certain
combination of relatively high W loading and low Nb loading, the
propene yield is maximized during ethylene
+ 2-butene
metathesis reaction in a fixed bed reactor. The best performing
catalyst W10Nb1-KIT-6 catalyst (W/Si = ~10 mol% and Nb/Si = ~1
mol%) displayed ~70% propene yield at steady state, which is
significantly higher than values previously reported with W-KIT-6
and W-EISA catalysts (~60%) under identical reaction conditions.
Nb incorporation enhances the dispersion of W species on the
catalyst surface and produces new active sites. Based on the H2-
TPD, XPS, EXAFS and ToF-SIMS results, the (O=)2W(O-Si)(O-
Nb) moiety is proposed as the possible active site precursor. In
contrast, transition elements such as Ta, Hf, and Zr perform less
effectively when doped into W-KIT-6. DFT computations predict
that the O=W=O bond angle is increased upon dopant (M)
introduction in the bimetallic WM-KIT-6 catalysts as follows:
Nb>Ta>Hf>Zr. This increase in O=W=O bond angle correlates
with enhanced metathesis activity on these catalysts compared to
W-KIT-6. We therefore hypothesize that the -O-Nb moeity tunes
the electronic environment around the W atom to increase the
metathesis activity.
Figure 10. Modelled active site precursor on W10Nb1-KIT-6 catalysts. The metal
structure is shown by the ball-and-stick-frame, the silica within the 6 Å sphere
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10.1002/cctc.201902131
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RESEARCH ARTICLE
filtrated, then dried at 100°C overnight. The resulting sample was
calcined in flowing air at 550°C for 5 h at a ramp rate of 1°C/min.
The catalysts are denoted as WxNby-KIT-6 where x and y
represents the W/Si and Nb/Si mole percentages in the synthesis
gel, respectively.
Methods
1.1 Materials
Ammonium metatungstate hydrate (H26N6O40W12·H2O, ≥ 85%
WO3 basis, gravimetric), Triblock copolymer Pluronic P123
(EO20-PO70-EO20, average MW
1.2.4 W10Hf1-KIT-6, W10Ta1-KIT-6, and W10Zr1-KIT-6 catalysts
= 5800), and tetraethyl
orthosilicate (TEOS, 98%), Pluronic F127 were purchased from
Sigma-Aldrich. Sodium tungstate (99.0-101.0%, Na2WO4·2H2O)
and WCl6 (99%) were purchased from Alfa Aesar. Niobium (V)
chloride (99+%-Nb) was purchased from Strem Chemicals. 1-
Butanol (HPLC grade), hydrochloric acid (certified ACS plus) and
ethanol were purchased from Fisher Scientific. 2-Butene (≥ 95
wt%, 39% cis-2-butene and 61% trans-2-butene) and ethylene (≥
99.5%) were purchased from Matheson Tri-Gas Inc. All the
reagents were used as received without further purification.
The synthesis of W10Hf1-KIT-6, W10Ta1-KIT-6, and W10Sn1-KIT-
6 catalysts are similar to the synthesis of W10Nb1-KIT-6 catalyst,
with the NbCl5 reagent being substituted with HfBr4, TaCl5 and
ZrOCl2, respectively.
1.2.5 WO3/Nb2O5 catalyst
The Nb2O5 is synthesized based on reported method[58] with
slight modification. The calcination of the as-synthesized Nb2O5
was carried out at 500°C. After calcination, Nb2O5 is impregnated
with required amounts of WCl6 solution in ethanol. The samples
were dried overnight at 100°C and calcined again at 500°C.
1.2 Catalyst preparation
1.2.1 W10-KIT-6 catalysts
1.3 Catalyst characterization
Following the protocols described in our earlier publication[15]
,
Small angle X-ray scattering and wide angle XRD patterns of
W-KIT-6, Nb-KIT-6, WNb-KIT-6 catalysts were analyzed in a
PANalytical Empyrean instrument, operating at 45 kV and 40 mA.
The samples prepared for TEM analysis were suspended in
W10-KIT-6 and W10-KIT-6 (Na) were synthesized by using
ammonium metatungstate hydrate and Na2WO4·2H2O as
tungsten source, respectively (Na represents Na2WO4·H2O). The
number 10 represents the W/Si mole percentage in the synthesis
gel. In a typical synthesis procedure of W10-KIT-6 catalyst, a
mixture of 4 g of P123, 140 mL of H2O and 6.1 g of 37.1 wt% HCl
was stirred at 38 ± 2°C overnight for dissolution. Then, 4.0 g of 1-
butanol were added and the mixture was stirred for 2.5 h.
Following this step, 8.7 g of TEOS was added and the stirring was
continued for 24 h. After that, 1.13 g of ammonium metatungstate
hydrate (dissolved in 5 ml of H2O) was added. After stirring for
another 24 h, the resulting mixture was transferred into a Teflon-
lined autoclave and heated to 100°C for 24 h. The solid product
was filtrated, then dried at 100°C overnight. The resulting sample
was calcined in flowing air at 550°C for 5 h at a ramp rate of
1 °C/min. The W10-KIT-6 (Na) catalyst synthesis was using
Na2WO4·2H2O as W source, TEOS and Na2WO4·H2O reagents
were added at the same time. Other procedures are same as W10-
KIT-6 synthesis.
ethanol solution and ultrasonically treated for
1
hour.
Approximately 5 µL of this mixture were dropped onto the copper
grid and dried in air, which was then transferred into the TEM
chamber for analysis. The catalyst morphology, particle size and
active sites were analyzed by bright-field and dark-field
transmission electron microscopy (TEM and STEM) using a FEI
Technai G2 transmission electron microscope, operating at an
electron acceleration voltage of 200 kV.
The W, Nb, and Si contents of the catalysts were measured by
inductively coupled plasma optical emission spectrometry (ICP-
OES) analysis on a Horiba Jobin Yvon JY 2000 instrument. The
nitrogen adsorption-desorption isotherms were obtained on a
Quantachrome NOVA 2000e instrument. The temperature-
programmed desorption of ammonia (NH3-TPD), and the
temperature-programmed reduction (H2-TPR) analysis were
performed on the Micromeritics Autochem 2910 instrument. The
diffuse reflectance UV/Vis (DR UV/Vis) spectra were obtained on
a PerkinElmer Lambda 850 spectrometer, using Spectralon as
the reference under ambient conditions. Detailed procedures for
the various techniques (ICP-OES analysis, N2 adsorption-
desorption isotherms, NH3-TPD, H2-TPR and UV/Vis) are
described elsewhere.[15,33,59]
1.2.2 Nb1-KIT-6 catalyst
The Nb1-KIT-6 catalyst with Nb/Si mole ratio percentage of 1
was synthesized by hydrothermal method according to previously
reported procedure.[38] 0.112 g of NbCl5 and 8.7 g of TEOS were
simultaneously added to a mixture of P123, H2O, HCl, and 1-
butanol. Other steps are identical to those used for synthesizing
W10-KIT-6 catalyst, with Nb source replacing the W source.
X-ray photoelectron spectroscopy (XPS) was measured on a
PHI 5000 Versa Probe II instrument. Monochromated Al Kα (50
W, 15 kV) X-ray and a beam diameter of 200 µm was used for the
measurement. The charging effect was eliminated by a dual-
beam charge neutralizer. The spherical capacitance analyzer was
operated with a pass energy of 46.95 eV for detailed scan. Prior
to the analysis, the surface of the catalyst was sputtered with Ar+
ions (2.0 keV) for 5 min to expose the elements that reside deep
in the pores of the catalyst. The following spectra were recorded:
survey spectrum, Si 2p, Nb 3d, and W 4f. The resulting spectral
data were processed with MultiPak software. All XPS peaks were
referenced to the Si2p peak at 103.3 eV. For solid-state NMR
experiment, the fresh catalyst was treated with the dry N2 (high
purity 99.99%, purified with a moisture trap) in a reactor at 550°C
for 24 h with ramping rate of 2°C/min. The sealed reactor was
transferred into an Ar glove-box and the sample was packed into
1.2.3 WxNby-KIT-6 catalysts
In a typical synthesis of WxNby-KIT-6 catalysts, 4 g of P123
were dissolved in 140 mL of H2O and 6.1 g of 37.1 wt% HCl by
stirring at 38 ± 2°C overnight. Then, 4.0 g of 1-butanol were added
and the mixture was stirred for 2.5 h. Following this step, 8.7 g of
TEOS and the required amounts of NbCl5 (to yield 0.25 – 10 Nb/Si
mol%) were added and the stirring was continued for 24 h.
Following this step, required amounts of ammonium
metatungstate hydrate (to yield 1 – 10 W/Si mol%) dissolved in 5
ml of H2O was added and the mixture was stirred for another 24
h. The resulting mixture was transferred into a Teflon-lined
autoclave and heated to 100°C for 24 h. The solid product was
1
a 4 mm Bruker rotor. Then, the sample was investigated by H
MAS NMR spectroscopy on a Bruker Avance III WB 400 MHz
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10.1002/cctc.201902131
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RESEARCH ARTICLE
NMR instrument at a spinning rate of 10.0 kHz. The 1H chemical
shifts were referenced to tetramethylsilane (TMS) and the
precision of the 1H chemical shift is ± 0.1 ppm.
the temperature of the catalyst bed. The catalyst was activated in
flow N2 (50 std cm3/min) at 550°C for 1 h, following which the
reactor was cooled down to desired reaction temperature. Then,
a mixture of 2-butene and ethylene gas was introduced into the
reactor. The metathesis reaction was performed at T = 450°C, P
= 1 atm, WHSV (ethylene and 2-butene) = 2.0 h-1, n(ethylene)/n(2-
butene) in feed = 3/1. At these operating conditions, the reaction
rate was confirmed to be not limited by either internal or external
transport limitations.[15] The products were analyzed by an on-line
Varian CP-3800 gas chromatograph, equipped with an Agilent
GS-gaspro® column (30 m x 0.320 mm) and a flame ionization
detector (FID). The transfer-line between the reactor and the six-
way valve was heated to 200°C to prevent product condensation.
A typical gas chromatograph based on sampling and analysis of
the reactor effluent stream is given in Figure S8. The various
catalyst assessment metrics are as follows[17]. The apparent
activity is defined as the number of moles of 2-butene converted
to propene per gram of catalyst per hour.[52]
The mass spectra were collected on an OmniStar GSD 320 O
mass spectrometer (Pfeiffer Vacuum). The range for the detection
range is 1 to 300 aum. The temperature for the transfer line is set
at 120°C to prevent condensation.
The time-of-flight secondary ion mass spectroscopy (ToF-SIMS)
results were collected on a ToF-SIMS V instrument (ION-TOF
GmbH Germany). The samples with 1×1 cm2 area were mounted
on a suitable sample holder and directly introduced into the
instrument. The Bi3+ primary ion was used for the analysis with a
primary ion energy of 30 keV and an analysis current of 0.75 pA.
The analysis area is 50 × 50 µm2 and the measurement time is
100 s. During the measurement, the flood gun was used for
charge compensation.
[2– ꢈꢉꢊꢋꢌꢋ]ꢍꢇ − [2– ꢈꢉꢊꢋꢌꢋ]ꢎꢄꢅ
ꢀꢁꢂꢃꢄꢅꢆꢇꢆ
=
× 100%
X-ray absorption spectroscopy (XAS) measurements were
used to identify the oxidation state, local symmetry, coordination
of tungsten and niobium. W L3-edge spectra were collected at the
IDEAS beamline of Canadian Light Source (CLS), using Ge (220)
double crystal monochrometer for an energy range of 3.4-13.4
keV. Nb L3-edge XAS spectra were collected at the SXRMB
beamline at the CLS. W L-edge spectra were recorded in both the
transmission and fluorescence modes and transmission spectra
were used for comparison. For Nb L-edge at the SXRMB, spectra
were recorded in fluorescence mode using an SDD detector, and
in total electron yield (TEY) by measuring the sample drain current.
For high concentration samples, e.g., Nb2O5, the TEY spectrum
is used while the fluorescence spectrum was used for catalysts
with low Nb loading. All measurements were performed at room
temperature using pelleted powders with appropriate thickness
for W L-edge or with powders spread thinly on the conducting
carbon tape in a vacuum chamber for Nb L-edge. All the XAS data
were analyzed by using Athena® software.
[2 − ꢈꢉꢊꢋꢌꢋ]ꢍꢇ
[ꢒꢓꢔꢒꢋꢌꢋ]ꢎꢄꢅ
ꢏꢐꢑꢎꢐꢆꢇꢆ
=
× 100%
[2– ꢈꢉꢊꢋꢌꢋ]ꢍꢇ − [2– ꢈꢉꢊꢋꢌꢋ]ꢎꢄꢅ
ꢕ
= ꢀꢁꢂꢃꢄꢅꢆꢇꢆ × ꢏꢐꢑꢎꢐꢆꢇꢆ × 100%
ꢐꢑꢎꢐꢆꢇꢆ
[ꢛꢎꢅꢜꢝ ꢞꢜꢑꢃꢎꢇ ꢍꢇ ꢐꢑꢎꢟꢄꢞꢅꢠ]
ꢖꢗꢘꢘ ꢈꢗꢙꢗꢌꢚꢋ ꢚꢙꢔꢘꢉꢓꢋ = [ꢛꢎꢅꢜꢝ ꢞꢜꢑꢃꢎꢇ ꢍꢇ ꢡꢆꢆꢟ ꢞꢎꢢꢐꢎꢇꢆꢇꢅꢠ] × 100%
where X2-butene, Spropene and Ypropene represent the steady state
values of 2-butene conversion, propene selectivity and propene
yield, respectively; WHSV, M2-butene and mcatalyst represent weight
hourly space velocity, molar mass of 2-butene and the mass of
catalyst, respectively.
1.5 DFT Calculations
The bimetallic amorphous silica clusters are built on the basis
of the monometallic clusters from METASIL, METal-doped
Amorphous Silicate Library, which was previously used to
reproduced experimental trends of acidity and NMR spectra.[60, 61]
The methodology for generating METASIL clusters is reported
elsewhere[60]. Twenty clusters have been taken from METASIL,
ten doped with Zr(IV) and ten with Nb(V), both including one
hydroxyl group on the metal. The bimetallic clusters are created
by adding a WO3 group between a metal hydroxyl and a nearby
silanol of a monometallic cluster (Schemes S1 and S2). Examples
of the active sites precursor of W10Nb1-KIT-6, W10Hf1-KIT-6,
W10Zr1-KIT-6, and W10Ta1-KIT-6 catalysts are shown in Figures
10 and S16. An additional ten monometallic W(VI) clusters are
taken directly from METASIL with no alterations to the structure
to compare with the bimetallic clusters. To maintain the
amorphous character of the silica, only a sphere with a radius of
6 Å centered at the non-tungsten metal is allowed to relax during
the geometry optimization. The structure is treated with the
B3LYP functional and D3 dispersion correction.[62] The basis sets
used are Def2TZVP and Stuttgart pseudopotentials for W, Nb, Ta,
Hf and Zr, 6-31++g(2d,p) for hydroxyl groups within the 6 Å
sphere, 6-31g(d) for the Si and O within the sphere, and 3-21g for
the surrounding frozen silica.[63] All calculations were performed
The Aspen Plus® 8.6 software was used to simulate the
equilibrium conversion of 2-butene and ethylene to propene at
different temperatures. The RCSTR reactor and Peng-Robinson
equation of state were selected for the calculation. The following
reactions were used for the calculation.
(1) ethylene + cis-2-butene D propene
(2) ethylene + trans-2-butene D propene
(3) cis-2-butene D 1-butene
(4) trans-2-butene D 1-butene
The software uses Gibbs free energy minimization technique
to compute the results [i.e., equilibrium conversions when the ΔG
of reactions (1-4) equal 0]. The predictions are unaltered as long
as the total molar amount of butenes (mixture of cis-2-butene,
trans-2-butene and 1-butene) is constant. As 2-butene is a
mixture of cis-2-butene and trans-2-butene, we simply use 1-
butene and ethylene as reagents for ease of computation.
with
a development version of the GAUSSIAN suite of
1.4 Metathesis activity
programs.[64]
Detailed procedures for the measurement of the metathesis
activity has been previously reported.[15, 17] Briefly, 1.0 g of catalyst
(0.425-0.850 mm) was loaded into the center of the stainless steel
tube reactor (i.d. = 9.4 mm). A thermocouple was used to detect
Acknowledgements
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10.1002/cctc.201902131
ChemCatChem
RESEARCH ARTICLE
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We gratefully acknowledge support from the joint National
Science Foundation and Environmental Protection Agency
program Networks for Sustainable Material Synthesis and Design
(NSF-EPA1339661). J.F.W. acknowledges support from the
National Natural Science Foundation of China (No. 21803027),
Fundamental Research Funds for the Central Universities
(lzujbky-2018-k08 and lzujbky-2019-cd02), and the Natural
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Entry for the Table of Contents
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RESEARCH ARTICLE
Synergistic Metal Doping Effects:
Niobium incorporation into W- KIT-6
materials significantly enhances
propene yield during 2-butene +
ethylene metathesis reaction.
Jian-Feng Wu*, Anand Ramanathan,
Reinhard Kersting, Amy Marie Jystad,
Hongda Zhu, Yongfeng Hu, Craig P.
Marshall, Marco Caricato, and Bala
Subramaniam*
Page No. – Page No.
Enhanced Olefin Metathesis
Performance of Tungsten and
Niobium Incorporated Bimetallic
Silicates: Evidence of Synergistic
Effects
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