The effect of supported MoOX structures on the reaction pathways of propene formation in the metathesis of ethylene and 2-butene

Article abstract of DOI:10.1039/c4cc01827c

The kind of surface MoO_X structures on Al₂O ₃-SiO₂ was found to determine propene selectivity in the metathesis of ethylene and 2-butene. Compared to isolated tetrahedral MoO _X species, their polymerized octahedral counterparts show significantly lower activity for isomerisation of 2- to 1-butene thus hindering non-selective metathesis of these butenes. In addition, they reveal higher ability to engage ethylene in propene formation. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc01827c

Showcasing research from the "Catalyst Discovery & Reaction
Engineering Department", Leibniz-Institut für Katalyse
e.V. an der Universität Rostock, Germany
As featured in:
The effect of supported MoO structures on the reaction
x
pathways of propene formation in the metathesis of ethylene
and 2-butene
Site requirements are the key! In their communication on
page 9060, T. Hahn, E. V. Kondratenko and D. Linke reveal the
importance of the kind of supported MoO structures for forming
x
propene with high selectivity in the metathesis of ethylene and
2-butene.
See T. Hahn, E. V. Kondratenko
and D. Linke, Chem. Commun.,
2014, 50, 9060.
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The effect of supported MoO structures on the
X
reaction pathways of propene formation in the
metathesis of ethylene and 2-butene†
Cite this: Chem. Commun., 2014,
0, 9060
5
Received 11th March 2014,
Accepted 30th April 2014
T. Hahn, E. V. Kondratenko* and D. Linke*
DOI: 10.1039/c4cc01827c
www.rsc.org/chemcomm
The kind of surface MoO
X
structures on Al
2 3
O
–SiO
2
was found to tetrahedral and polymerized octahedral MoO species is also an
X
8
determine propene selectivity in the metathesis of ethylene and essential activity-determining factor.
-butene. Compared to isolated tetrahedral MoO species, their
polymerized octahedral counterparts show significantly lower (i) the differences between supported tetrahedral and octahedral
activity for isomerisation of 2- to 1-butene thus hindering non- MoO species in their selectivity to propene in the metathesis of
selective metathesis of these butenes. In addition, they reveal ethylene and 2-butene and (ii) the fundamental roots of these
2
X
Based on the above background, we disclose for the first time
X
higher ability to engage ethylene in propene formation.
differences. Despite a similar intrinsic activity (Table S1 in ESI†),
octahedral MoO species show propene selectivity above 90%
X
Propene is a versatile building block in the chemical industry.
Since the last twenty years its demand has continuously increased
especially owing to the increasing production of polypropylene.
compared to only 65% achieved over tetrahedral MoO species at
X
similar ethylene conversion (10–12%) using a stoichiometric
ethylene/trans-2-butene feed (Fig. 1). To explain these differences
in selectivity, we investigated this reaction in a broad range of
conversion of feed components with ethylene/trans-2-butene
feeds varying in the ratio between 0 and 2.8 (Table S2 in the
ESI†). Prefix trans is only used for feed compositions, while
1
To accommodate the propene demand, the metathesis of ethylene
and 2-butene to propene over a WO /SiO catalyst has been
X
2
2–4
commercialized.
However, the selectivity of this process is
hampered by the co-production of pentenes through metathesis
5
of 1- and 2-butenes. 1-Butene may either be introduced as part of
2-butene stands for cis- and trans-2-butenes, because the former
4
the feed, which is typically derived from the C -cut of a steam
was not considered a reaction product. In addition, we system-
atically analysed possible molybdenum carbene species that may
form from feed components and reaction products under con-
cracker, or may be formed via 2- to 1-butene isomerisation over
the metathesis catalyst. In order to achieve high propene selectivity,
the metathesis reaction is industrially performed using a feed with
12
sideration of a metallacyclobutane intermediate mechanism.
6
an excess of ethylene. From an economic viewpoint, operation
using an ethylene-rich feed is not optimal, because of lower
propene productivity than using a stoichiometric feed. To increase
the productivity, it is essential to design catalysts producing
propene as selective as possible. For this purpose, the origins
governing propene selectivity must be comprehended.
To the best of the authors’ knowledge, such studies have not
been reported up to now, while activity-determining factors have
been thoroughly investigated. For example, the rate of propene
formation over Mo-containing catalysts, which are promising
7
,8
alternatives to WO
X 2
/SiO , is influenced by (i) the support and
9–11
(ii) the structure of surface MoO
X
species.
Recently, it has
been demonstrated that Brønsted acidity of highly dispersed
Leibniz-Institut f u¨ r Katalyse e.V. an der Universit a¨ t Rostock, Albert-Einstein-Str. 29a,
Fig. 1 Selectivity (S) to propene in the metathesis of ethylene and trans-
1
8059 Rostock, Germany. E-mail: Evgenii.Kondratenko@Catalysis.de,
David.Linke@Catalysis.de
Electronic supplementary information (ESI) available: Additional experimental
section, tables and figure. See DOI: 10.1039/c4cc01827c
2-butene over tetrahedral (’) and octahedral ( ) MoO
X
species on
Al –SiO supports with different content of SiO . Reaction conditions:
2
O
3
2
2
†
423 K, 125 kPa and ethylene/trans-2-butene/nitrogen = 5/5/1. Ethylene
conversion was between 0.10 and 0.12.
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Catalysts were prepared by incipient wetness impregnation of
s
2 3 2
Al O –SiO supports (Siral , Sasol) with aqueous solutions of
ammonium heptamolybdate (Riedel-de Haen). They are denoted
À2
XMoSY, where ‘‘X’’ stands for the nominal Mo loading (Mo nm
)
and ‘‘Y’’ for the SiO content (wt%) of the bare support. Supports
2
2
and catalysts were characterized by N adsorption, ICP-OES, XRD,
UV-vis and Raman spectroscopy, H -TPR and IR pyridine adsorp-
2
8
tion. The obtained results revealed that all catalysts with 0.15 Mo
nm possess highly dispersed tetrahedral MoO
those with 1.5 Mo nm contain polymerized octahedral MoO
À2
X
species, while
À2
X
.
Scheme 1 Catalytic cycles of the metathesis of (A) ethylene and 2-butene
1
2
and (B) 1- and 2-butene according to Chauvin.
Catalytic tests were carried out in fixed-bed reactors at 423 K and
25 kPa. Further experimental details are given in ESI.†
To derive mechanistic insights into the effect of catalyst on 2-butene isomerisation, while dimerization of ethylene to
1
propene selectivity presented in Fig. 1, we analysed how the 1-butene can be excluded because the latter olefin was not
selectivity to the target and side products depends on the degree observed in a separate test with an ethylene/nitrogen = 1/16
of 2-butene conversion measured at contact times between 0.03 feed. 2- to 1-Butene isomerisation is significantly faster than
À1
and 1.10 s g ml . Such tests were performed with a standard metathesis of 1- and 2-butenes to propene as evidenced by the
ethylene/trans-2-butene/nitrogen = 5/5/1 feed over 0.15MoS70 and high selectivity to 1-butene at low conversion of 2-butene. The
1.5MoS10 representing the catalysts with tetrahedral and octa- low selectivity to pentene (Fig. 2), which should be formed in
hedral MoOX species, respectively. This selectivity-conversion similar amounts as propene through metathesis of 1- and
analysis helps to elucidate if and how the target reaction is 2-butenes, may be related, for example, to its metathesis with
influenced by side reactions like 2- to 1-butene isomerisation, ethylene to propene and 1-butene.
metathesis of these butenes or metathesis of ethylene and
As the surface of our catalysts is only partly covered by MoO
species, we also tested the corresponding bare supports to check
X
5,6,13,14
pentene.
The obtained results are summarized in Fig. 2.
One can clearly see that the propene selectivity over 1.5MoS10 is their activity and selectivity. Both supports did not show any activity
around 97% at 10% 2-butene conversion and does not practically for propene formation but catalysed 2- to 1-butene isomerisation
change with increasing conversion. From a mechanistic viewpoint, with a similar activity (Table S3 in the ESI†). Importantly, 1-butene
this selectivity–conversion relationship implies that propene is was not observed over 1.5MoS10 at low 2-butene conversion, while
directly formed from 2-butene through its metathesis with ethylene significant amounts were detected over 0.15MoS70. This implies
(
Scheme 1, A). This reaction scheme is not valid for 0.15MoS70 as that the isomerisation rate (formation of 1-butene) over the latter
concluded from the fact that the selectivity to propene is strongly catalyst is higher than the rate of 1-butene conversion through
influenced by 2-butene conversion. It increases from 44 to 79% with metathesis with 2-butene as otherwise 1-butene would not be
a corresponding increase in the conversion from 16 to 47%, but is observed. To estimate the contribution of the bare support to the
always below the values obtained over 1.5MoS10. Since the increase isomerisation activity of 0.15MoS70, we calculated the rate of
in propene selectivity is coupled with a simultaneous decrease 1-butene formation; it is approximately 3.5 times higher over the
in 1-butene selectivity, we conclude that metathesis of 1- and catalyst than over the bare support (Table S3 in the ESI†). Bearing
2
0
-butenes strongly contributes to propene formation over in mind that 0.15MoS70 also consumed 1-butene via metathesis
.15MoS70 (Scheme 1, B). 1-Butene is primarily formed via with 2-butene, the difference in the isomerisation activity between
the catalyst and the bare support must be even higher. Thus, 2- to
1
-butene isomerisation is predominantly and propene formation is
exclusively catalysed by MoO . Consequently, the selectivity-
conversion relationship presented in Fig. 2 is primarily determined
by the kind of MoO and not by the support. Considering the
mechanism of olefin metathesis via molybdacyclobutane inter-
X
X
12
mediates, the effect of the MoO structure (tetrahedral vs. octa-
X
hedral) on propene selectivity can be governed by (i) the formation
of the suitable molybdenum carbene species (MoQCHR), (ii) the
chemisorption of olefins on MoO species leading to MoQCHR
X
and (iii) the rates of formation of molybdacyclobutane intermediates
and their decomposition to gas-phase metathesis products.
To derive insights into the kind of MoQCHR required for
selective propene formation, we have systematically analysed
the possible reaction pathways occurring in the metathesis of
ethylene and 2-butene. Considering the mechanism of propene
metathesis on isolated MoOX, we put forward that different
primarily MoQCHR species are formed upon reaction of
Fig. 2 Selectivity (S) to propene (’, ), 1-butene (K, ), pentene (m,
)
and hexene (., ) versus conversion (X) of 2-butene over 0.15MoS70
solid symbols) and 1.5MoS10 (open symbols). Reaction conditions: 423 K,
25 kPa and ethylene/trans-2-butene/nitrogen = 5/5/1.
1
5
(
1
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isomerization precedes metathesis of 1- and 2-butenes. Propene
was formed over both catalysts, however, five times faster over
0
.15MoS70 than over 1.5MoS10 (Table S5 in the ESI†). Moreover,
at low 2-butene conversion, 1-butene was detected only over
.15MoS70. Taking into account this information and similar
Scheme 2 Initial MoQCHR generated by (C) ethylene, (D) 2-butene intrinsic activity of both catalysts for 2- and 1-butenes metathesis
including the isomerisation pathway of 2- to 1-butene) and (E) 1-butene.
0
(
(Table S4 in the ESI†), we can safely conclude that 0.15MoS70
shows significantly higher isomerisation activity than 1.5MoS10.
ethylene and 2-butene with reduced MoO
in Scheme 2, C and D, respectively. This scheme additionally catalyst.
X
(Mo) sites as shown This may be related to higher Brønsted acidity of the former
8
illustrates the formation of 1-butene via isomerisation (E) and
its subsequent reaction with reduced Mo sites (F).
Another important difference between isolated tetrahedral and
polymerized octahedral MoOX species is their ability to engage
At low degrees of 2-butene conversion, i.e. low amounts of ethylene in propene formation. This conclusion was derived from
reaction products, feed olefins preferentially react with MoQCHR analysing the reaction rates of propene formation (rMo(propene))
formed from the feed components (pathways F–K in Scheme 3). and ethylene consumption (rMo(ethylene)) in catalytic tests using
Upon rising 2-butene conversion, secondary reactions involving reaction feeds with an ethylene/trans-2-butene ratio between 0.1
reaction products and additionally formed MoQCHR can result in and 2.8. With increasing partial pressure of ethylene, a rising
propene (L–O in Scheme 3). Its formation is only possible when propene selectivity was observed (Fig. S1 in the ESI†). From a
2 2 4
MoQCH and MoQC H are involved in catalytic cycles (F, H and kinetic viewpoint, rMo(propene) is a sum of rates of metathesis
L–O in Scheme 3). Such metal carbene species are exclusively of 1- and 2-butenes and ethylene-assisted metathesis, while
formed with participation of ethylene. Furthermore, a 100% selec- rMo(ethylene) represents the latter process exclusively. Consequently,
tive propene production is only achieved if F is alternating with L the ratio of rMo(propene)/rMo(ethylene) provides the contribution
(
Scheme 1, A). Considering this knowledge and the observed of metathesis of 1- and 2-butenes to overall propene formation. If
propene formation over 0.15MoS70 and 1.5MoS10, both catalysts this metathesis reaction did not run, the ratio of rMo(propene)/
should possess MoQCH and MoQC species. Therefore, we rMo(ethylene) would be equal to 2, which is typical for selective
can conclude that the MoO structures have no principle influence metathesis of ethylene and 2-butene to propene (Scheme 1, A). Fig. 3
2
2 4
H
X
on the ability to generate propene-forming carbene species. illustrates this ratio as a function of partial pressures of ethylene and
However, what are the fundamental origins for the different 2-butene. For 1.5MoS10, it decreases from 2.4 to 2.1 with an increase
propene selectivity of 0.15MoS70 and 1.5MoS10 in Fig. 1 and 2?
in the feed ratio of ethylene/trans-2-butene from 0.1 to 0.4 and
To clarify the above question, we analysed the ability and remains constant upon further increasing partial pressure of
activity of 0.15MoS70 and 1.5MoS10 (i) to generate propene via ethylene. This means that propene is mainly produced through
metathesis of 1- and 2-butenes (Scheme 1, B) and (ii) to catalyse the metathesis of ethylene and 2-butene. This reaction is
2- to 1-butene isomerisation. Catalytic tests with a trans-2-butene/ significantly faster than the metathesis of 1- and 2-butenes
1-butene/nitrogen (5/5/1) feed revealed that both catalysts produce probably due to the low 2- to 1-butene isomerisation activity.
propene with similar activity and selectivity (Table S4 in the ESI†). Contrarily to 1.5MoS10, 0.15MoS70 containing isolated tetrahedral
These rates are also close to those in the metathesis of ethylene MoOX species generates propene faster from butenes than from
and 2-butene. To indirectly check the isomerisation activity of ethylene and 2-butene. A value of rMo(propene)/rMo(ethylene) close to
0.15MoS70 and 1.5MoS10, we carried out catalytic tests using a 2 was only achieved when using a feed with 2.8 times excess of
trans-2-butene/nitrogen (1/16) feed. Under this feed 2- to 1-butene ethylene over trans-2-butene. Moreover, rMo(propene)/rMo(ethylene)
strongly increased with a decrease in ethylene partial pressure
and reached 5.3 for the feed with an ethylene/trans-2-butene ratio
of 0.1. These results evidence significantly lower activity (or lower
concentration) of MoQCHR species formed from tetrahedral MoOX
species for converting ethylene compared to those originated from
octahedral MoOX.
Based on the above discussion, we put forward that in
addition to 2- to 1-butene isomerisation, the structure of MoO
X
influences (i) the rates of formation of MoQCHR species from
ethylene and butenes or their decomposition and (ii) the rates
of reacting gas-phase olefins with MoQCHR to produce moly-
bdacyclobutane intermediates or to decompose them to other
gas-phase olefins. The latter assumption is indirectly supported
by previous DFT studies on the initial steps of ethylene inter-
Scheme 3 (F–K) overall primary metathesis pathways and (L–O) second-
ary metathesis pathways only for propene formation. For simplification, we
action with MoQCH
2
formed from monomeric and dimeric
1
6
reduced MoO supported on Al
X
2
O
3
.
The reactivity of indivi-
4 8
do not distinguish between isomeric structures of MoQC H and isomers
of pentene and hexene.
dual sites was predicted to depend on their geometry and
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with participation of ethylene leading to propene. Compared to
these species, metathesis of ethylene and 2-butene is signifi-
cantly faster than other side reactions over octahedral MoOX
species. This is the reason for their high propene selectivity,
which was even achieved under strong excess of 2-butene over
ethylene. For deriving further fundamental details about the
kinetics and the interaction of MoQCHR with ethylene and
X
2-butene over differently structured MoO species, we are
currently investigating these processes by means of in situ
infrared spectroscopy and pulse experiments.
Financial support from Deutsche Forschungsgemeinschaft
(DFG, project KO 2261/3-1, Li 2047/1-1) is gratefully acknowledged.
Fig. 3 Ratio of the propene formation rate related to the rate of ethylene
consumption over (’) 0.15MoS70 and ( ) 1.5MoS10 at different ratios of
partial pressure (p) of ethylene to trans-2-butene at 423 K, 125 kPa total
pressure and a conversion of 2-butene ranging between 0.13 and 0.44.
The sum of the partial pressure of ethylene and trans-2-butene was kept
between 0.96 and 1.14.
Notes and references
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3
4
5
G. Parkinson, Chem. Eng., 2001, 108, 27–35.
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X
electronic structure. For monomeric MoO sites, the formation
of a stable molybdacyclobutane intermediate is kinetically
favoured over the cycloreversal step into metathesis products,
while the latter step is preferred on dimeric sites. These results
might explain the low ability of 0.15MoS70 with isolated tetra-
hedral MoOX to form propene with participation of ethylene.
However, to draw a definitive conclusion about the reactivity
6 R. J. Gartside and M. I. Greene, US Pat., 7,214,841 B2, 2007.
7
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9
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are required.
1
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In summary, the results reported here demonstrate for the
1
1
first time that the kind of supported MoO
X
species is a key
factor governing propene selectivity in the metathesis of
ethylene and 2-butene. This is due to the fact that tetrahedral 14 D. Zhang, X. Li, S. Liu, X. Zhu, F. Chen and L. Xu, Appl. Catal., A,
2
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pathways. 2- to 1-Butene isomerisation and metathesis of
1
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X
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