The M1 phase of MoVTeNbO as a catalyst for olefin metathesis and isomerization

Article abstract of DOI:10.1002/cctc.201402608

Polycrystalline MoVTeNb oxide composed of the M1 phase was studied in the transformation of propene at reaction temperatures between 323 and 573 K in the absence of oxygen and water. Surprisingly, the catalyst showed considerable activity in propene metathesis to ethene and 2-butenes, which was attributed to partial reduction of the catalyst surface under the reaction conditions. The multifunctionality of the M1 phase, which is known as a selective catalyst for the direct oxidation of propane to acrylic acid, was furthermore reflected in the formation of 1-butene and isobutene owing to C4 isomerization. This study demonstrates the diversity of the M1 phase as a catalyst, which is based on surface dynamics and structural stability, and further improves the understanding of the complex reaction network of short-chain hydrocarbons over MoVTeNbO M1 catalysts.

Full text of DOI:10.1002/cctc.201402608

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DOI: 10.1002/cctc.201402608
The M1 Phase of MoVTeNbO as a Catalyst for Olefin
Metathesis and Isomerization
Kazuhiko Amakawa, Yury V. Kolen’ko, Robert Schlçgl, and Annette Trunschke*[a]
Polycrystalline MoVTeNb oxide composed of the M1 phase was
studied in the transformation of propene at reaction tempera-
tures between 323 and 573 K in the absence of oxygen and
water. Surprisingly, the catalyst showed considerable activity in
propene metathesis to ethene and 2-butenes, which was at-
tributed to partial reduction of the catalyst surface under the
reaction conditions. The multifunctionality of the M1 phase,
which is known as a selective catalyst for the direct oxidation
of propane to acrylic acid, was furthermore reflected in the for-
mation of 1-butene and isobutene owing to C4 isomerization.
This study demonstrates the diversity of the M1 phase as a cat-
alyst, which is based on surface dynamics and structural stabili-
ty, and further improves the understanding of the complex re-
action network of short-chain hydrocarbons over MoVTeNbO
M1 catalysts.
text, we explored the conversion of propene over the M1
phase in the absence of oxygen. Surprisingly, we discovered
that the MoVTeNbO M1 phase performs the metathesis of pro-
pene to form ethene and 2-butenes at a temperature well
below that of typical alkane oxidation conditions.
Appreciable conversion of propene into ethene and butenes
in an approximately 1:1 ratio was observed above 423 K (Fig-
ure 1a); this suggests the occurrence of propene metathesis
Crystalline MoVTeNbO materials containing the orthorhombic
M1 phase (ICSD no. 55097)[1] as a major component represent
the most active and selective catalysts for the direct oxidation
of light alkanes.[2,3] The M1 phase alone is able to accomplish
the direct oxidation of propane to acrylic acid, which involves
abstraction of four hydrogen atoms, addition of two oxygen
atoms, and exchange of eight electrons.[2,4,5] The multiple cata-
lytic functions of the M1 phase capable of activating various
functional groups (e.g., alkanes, alkenes, alcohols, aldehydes)
are considered a key aspect that enables the direct formation
of acrylic acid with high selectivity.[6,7] Structural assignment of
the catalytic functions is not straightforward, as the catalytic
functions of M1 are closely related to the dynamics of the M1
surface changes in response to the reaction atmosphere (e.g.,
gas composition, temperature).[8–10] It is likely that the gradient
of the gas composition within the catalyst bed influences the
catalytic properties of M1. Especially, reaction conditions with
a regulated oxygen partial pressure are often applied in pro-
pane oxidation to maximize the yield of the desired prod-
ucts.[11] The formation of an “oxygen-deficient zone” inside the
catalyst bed becomes plausible under such conditions, and
this leads to extended modification of the catalyst surface that
potentially impacts the catalytic performance.[12] In this con-
Figure 1. Propene reaction over the MoVTeNbO M1 catalyst at 323–573 K:
a) Formation rate of ethene and butenes; b) formation of C3H6O (m/z=58)
monitored by mass spectrometry.
activity. The consumption rate of propene over the bulk MoV-
TeNbO M1 catalyst, which is characterized by a low specific
surface area (SBET =7.5 m2 gÀ1), reached a remarkable value of
1 mmolpropene gcatÀ1 hÀ1 at the highest reaction temperature of
573 K. This is close to the temperature range in which the cata-
lyst normally operates in selective oxidation reactions. The ac-
tivity was, however, lower than the specific activity of molyb-
denum oxide species anchored on the surface of supports
with high specific surface area, which are typically applied in
olefin metathesis reactions; these catalysts include MoOx/SiO2
(8 mmolpropene gcatÀ1 hÀ1 at 323 K, SBET =556 m2 gÀ1)[13] and MoOx/
[a] Dr. K. Amakawa, Dr. Y. V. Kolen’ko,+ Prof. Dr. R. Schlçgl, Dr. A. Trunschke
Department of Inorganic Chemistry
Fritz-Haber-Institut der Max-Planck-Gesellschaft
Faradayweg 4–6, 14195 Berlin (Germany)
Fax: (+49)30-8413-4405
E-mail: trunschke@fhi-berlin.mpg.de
[+] Present address:
SiO2–Al2O3
(30 mmolpropene gcatÀ1 hÀ1
at
313 K,
SBET =
International Iberian Nanotechnology Laboratory
Braga 4715-330 (Portugal)
480 m2 gÀ1).[14] Upon normalizing the rates at 323 K to the sur-
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face area, the activity of the MoVTeNbO M1 catalyst
(1 mmolpropene mcatÀ2 hÀ1) is one order of magnitude lower than
that of MoOx/SiO2 (14 mmolpropene mcatÀ2 hÀ1).[13] The ethene/bu-
tenes ratio remained at approximately 1 with increasing tem-
peratures up to 573 K, and this indicates that olefin conversion
is predominantly the result of propene self-metathesis to yield
ethene and 2-butenes. In addition to the metathesis reaction,
subsequent rearrangement of the 2-butenes into 1-butene and
isobutene was suggested (Figure 2). The selectivity profile in
Scheme 1. Suggested pathway for the formation of active Mo–carbene sites
in the propene reaction on the surface of MoVTeNbO M1.
the vanadium centers and, second, charge transfer to the mo-
lybdenum centers, which leads to the formation of MoIV cen-
ters. Alternatively, molybdenum oxide sites may be directly in-
volved. Te4+ÀO and O=Mo6+=O centers in close vicinity on the
surface of M1 have been considered to be able to perform a-H
abstraction from adsorbed propene and O insertion into the
resulting p–allylic intermediate, respectively, which lead to
Mo4+ and desorption of acrolein.[19]
Reduced molybdenum centers formed on the M1 surface
under the reaction conditions are likely responsible for the ob-
served C4 isomerization activity (i.e., formation of 1-butene
and isobutene). Supported molybdenum oxide catalysts in re-
duced states are known to be active for C4 isomerization reac-
tions at 623 K,[20] for which Brønsted acidity has been suggest-
ed to play an important role.[21] Indeed, we found that the
MoVTeNbO M1 catalyst exhibits Brønsted acidity,[8] which may
contribute to the C4 isomerization activity.
The reduction of the M1 surface by propene was also sup-
ported by a temperature-programmed reaction (TPR) with pro-
pene (Figure 3). The broad band of C3H6O (m/z=58) centered
Figure 2. a) Product distribution of the propene reaction over the MoVTe-
NbO M1 catalyst at 323–573 K and b) suggested reaction scheme for pro-
pene conversion.
Figure 2a reveals the formation of a minor amount of 1-
butene and a significant increase in isobutene selectivity at
elevated temperatures. At the highest reaction temperature of
573 K, the relative ratio of the butenes approaches thermody-
namic equilibrium.[15] Accordingly, it is suggested that the M1
phase exhibits multifunctional catalysis including olefin meta-
thesis (propene to ethene and 2-butenes), shift of the olefinic
bond (2-butenes to 1-butene), and skeletal isomerization (n-
butenes to isobutene).
Mo–carbene species (Mo=CHR) are known as active sites
for olefin metathesis.[16] The occurrence of metathesis activity
over M1 suggests the generation of Mo–carbene species on
the M1 surface through the reaction of the M1 surface and
propene under anaerobic conditions. The development of cat-
alytic activity above 423 K (Figure 1a) is accompanied by the
temporal formation of C3H6O (Figure 1b, acetone and/or
propanal), which suggests that the reduction of the catalyst
surface by propene (i.e., formation of C3H6O) is connected to
catalytic activity. Most likely, MoIV species formed by the oxy-
genation of propene act as precursors of the Mo–carbene spe-
cies (Scheme 1), as coordinatively unsaturated MoIV species
formed by surface reduction are known to be good precursors
of Mo–carbene species.[13,17,18] As vanadium is considered to be
essential for the oxidation activity of M1,[2,8] it is speculated
that the formation of MoIV involves, first, propene oxidation at
Figure 3. Profile for the consumption of propene (m/z=42) as well as the
profiles for the formation of acrolein/butene (m/z=56) and acetone/propa-
nal (m/z=58) traced by mass spectrometry during the temperature-pro-
grammed reaction of MoVTeNbO M1 in 5% propene at 2 KminÀ1
.
at 470 K is consistent with the temporal formation of C3H6O
detected upon increasing the reaction temperature in the cata-
lytic test (Figure 1b). A large consumption of propene with
concurrent formation of oxygenated products was observed at
approximately 750 K (which is much higher than the tempera-
ture range of the catalytic test) in the TPR profile (Figure 3);
this is indicative of bulk reduction of the M1 catalyst by pro-
pene, which probably yields bulk metal carbide materials.[22]
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Structural characterization of the catalyst used in metathesis
(Figure 2) indicated that the M1 phase preserved its bulk struc-
ture but that it did undergo a minor modification that was
much less severe than the reduction of M1 in propane at
673 K.[10] No essential change was observed in the XRD pattern
after the propene reaction (Figure 4a), and this indicates that
the bulk structure of the M1 phase was preserved. A whole
erate under alkane oxidation conditions (T=613–693 K),[2] al-
though the degree of reduction of the M1 surface is very likely
lower than that of the present test conditions and, thus, lower
olefin isomerization activity is expected. The propene con-
sumption rate reached 130–85 mmolmÀ2 hÀ1 at 573 K, which is
comparable to typical propene formation rates in propane oxi-
dation over MoVTeNbO catalysts (e.g., 280 mmolmÀ2 hÀ1 at
653 K[5]). Accordingly, a minor amount of olefin isomerization
under the oxidation conditions seems plausible. Indeed, we
observed the formation of ethene and isobutene as minor
products under the propane oxidation conditions.[10] It is
speculated that the major fraction of the C4 olefins formed by
the metathesis route under the propane oxidation conditions
undergoes further oxidation to yield stable end products (e.g.,
CO2, methacrylic acid, maleic anhydride). Though it is not clear
how significant olefin isomerization is under propane oxidation
conditions, the present findings provide new insight into the
complex reaction network of propane oxidation over MoVTe-
NbO M1 catalysts and the versatility of the M1 phase in cataly-
sis.
In summary, we discovered propene metathesis and isomeri-
zation catalysis over the MoVTeNbO M1 catalyst to yield
ethene and butenes. It appears that the reduction of the M1
surface triggers the catalyst’s activity for metathesis and the
isomerization of olefins. The findings highlight the highly mul-
tifunctional nature of M1 and reveal the prominent role of dy-
namic changes in the surface of the catalyst under the reaction
conditions.
Experimental Section
Figure 4. Characterization of MoVTeNbO M1 before and after the propene
reaction. a) XRD patterns (the inset shows a magnification); b) Raman spec-
tra upon excitation at l=633 nm.
The phase-pure MoVTeNbO M1 catalyst (SBET =7.5 m2 gÀ1, internal
ID# 6902) was synthesized by spray drying and subsequent purifi-
cation. Details of the synthesis and characterization are reported
elsewhere.[5] Powder X-ray diffraction (XRD) analysis was performed
by using a STOE STADI-P transmission diffractometer equipped
with CuKa1 radiation. Confocal Raman spectra were collected at
room temperature by using a Horiba–Jobin Ybon LabRam instru-
ment equipped with red laser excitation (633 nm/1.96 eV, 1.5 mW
at the sample position). Propene conversion activity was tested in
a fixed-bed reactor at 323–573 K at a contact time of 0.15 sgmLÀ1
by using thoroughly dehydrated and deoxygenated neat propene
at atmospheric pressure. Inlet and outlet gases were analyzed by
an online gas chromatograph equipped with a flame ionization de-
tector. The activity is presented as the formation rates of the prod-
ucts normalized by the specific BET surface area of the catalyst. Se-
lectivity was calculated on the basis of the number of carbon
atoms in the products. Temperature-programmed reaction with
propene was performed with a fixed-bed reactor equipped with
a quadrupole mass spectrometer by using 5% propene in helium
powder pattern fitting indicates a very minor but notable
change in the lattice parameters of the a and b axes, which is
reflected in the shift of the (600) reflection at 2q=25–268 (Fig-
ure 4a, inset). The minor change in the lattice parameters is in-
dicative of the reduction of the M1 phase.[10] In agreement
with the XRD results, after the propene reaction the Raman
spectrum kept the general features of M1 but lost distinct
bands at n˜ =875 (MÀO stretching) and 450 cmÀ1. These two
Raman bands were identified as indicators of the high crystal-
linity of the M1 phase, as the bands occurred upon annealing
nanocrystalline M1 into microcrystalline M1 at elevated tem-
peratures.[23] Besides, no sign of coke deposition[24] at n˜ =
1300–1800 cmÀ1 was observed in the Raman analysis. Summa-
rizing the structural analyses, it is suggested that the M1 cata-
lyst underwent partial reduction of the M1 phase that involved
cleavage of the MÀO bonds, whereas the bulk crystal structure
was preserved. The preservation of the M1 bulk phase is likely
important for catalysis, because crystalline MoO3 showed no
propene conversion activity even after reductive pretreatment
by hydrogen or propene (data not shown).
at a heating rate of 2 KminÀ1
.
Acknowledgements
We thank Dr. Frank Girgsdies and Jasmin Allan for XRD experi-
ments and Dr. Raoul Naumann d’Alnoncourt for discussions. K.A.
is grateful to Mitsubishi Gas Chemical Co., Inc. for a fellowship.
Considering the significant activity observed at low tempera-
ture (i.e., T=423–573 K), olefin isomerization catalysis may op-
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Published online on && &&, 0000
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M1 FTW! The MoVTeNbO M1 phase,
known as an excellent catalyst for the
direct oxidation of propane to acrylic
acid, exhibits remarkable catalytic activi-
ty in the conversion of propene into
ethene and butenes. This activity is at-
tributed to partial reduction of the cata-
lyst surface under the reaction condi-
tions. The multifunctionality of the M1
phase is also reflected in the formation
of 1-butene and isobutene owing to C4
isomerization.
K. Amakawa, Y. V. Kolen’ko, R. Schlçgl,
A. Trunschke*
&& – &&
The M1 Phase of MoVTeNbO as
a Catalyst for Olefin Metathesis and
Isomerization
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ChemCatChem 0000, 00, 1 – 4
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