Highly selective and stable propane dehydrogenation to propene over
dispersed VOx-species under oxygen-free and oxygen-lean conditions
O. Ovsitser1, R. Schomaecker1, E. V. Kondratenko2, T.Wolfram3, A.Trunschke3
1
Technical University of Berlin, Straße des 17, Juni, 124-128, 8, D-10623, Berlin, Germany
2
Leibniz -Institute for Catalysis at the University of Rostock, Albert-Einstein-Str. 29a, D-
18059 Rostock, Germany
3
Department of Inorganic Chemistry, Fritz Haber Institute of the Max Planck Society,
Faradayweg 4-6, 14195 Berlin, Germany
Corresponding author: Olga.Ovsitser@campus.tu-berlin.de
Abstract
For the first time, we reported on a highly selective propane dehydrogenation to propene over
SiO2-supported dispersed VOx species under oxygen-free and oxygen-lean conditions.
Propene selectivity above 80 % was achieved with propane conversions above 45% at 823843 K. At operation under oxygen-lean conditions, consecutive conversion to COx was
significantly suppressed and propene was formed additionally via the highly selective nonoxidative dehydrogenation route over the same catalyst. Usage of SiO2 supports (MCM-41,
SBF-15, SiO2) with no strong acid sites and co-feeding of small O2 amounts lead to very
limited coke formation (< 2wt. % after 20 hours on-stream performance) under O2-lean
conditions. This results in a stable time-on-stream performance.
1.
Introduction
Functionalization of cheap and, as oil reserves run out, longer available C2-C5 alkanes from
natural gas towards olefins currently produced from oil-derived feeds, is a challenging topic
in catalysis. In our previous contribution 1 we demonstrated that combining oxidative and
non-oxidative dehydrogenation of iso-butane in one reactor system can significantly suppress
consecutive iso-butene conversion to COx at high iso-butane conversions. As a result, high
Corresponding author:
Phone: +49-30-31425644, Fax: +49-30-31425644, E-mail: Olga.Ovsitser@campus.tu-berlin.de
1
iso-butene selectivity and yields were obtained. In the present contribution we elaborated this
approach further and applied it for propane dehydrogenation to propene. Supported catalysts
with well-defined surface VOx species were used as catalysts.
2. Experimental
Highly-dispersed surface VOx species were prepared by impregnating differently structured
pure silica supports (MCM-41, SBF-15, SiO2) and MCM-41 modified with TiO2 with a
solution of vanadium acetylacetonate in toluene. Comments of preparation of VOx(4)/SBF-15
can be added here. The vanadium content and Ti/Si molar ratio were varied from 1 to11 wt.%
and from 0 to 1.5, respectively. Details of catalysts preparation and their thorough
characterization (TEM, TPR, in situ UV/Vis, in situ Raman) have been reported previously
2-4 (4-reference on VOx(4)/SBF-15 preparation and characterization). The presence of only
highly dispersed VOx species (no three-dimensional VOx-aggregates) was proven for the
sample, having up to 5 wt.% of V. The samples are denoted here by their amount of vanadium
loading (e.g. sample with V loading of 5.3 wt%, designated as VOx(5)/MCM-41). Catalytic
tests were performed with low (0-0.1) O2/C3H8 ratios and contact times (W/F) of 0.003-18 g s
ml-1 at temperatures of 773 - 843 K in a fixed-bed quartz reactor at atmospheric pressure,
using 0.005 - 0.6 g of catalyst. The feed components and the reaction products were analysed
by an on-line GC (HP 5890-II) using a Poraplot Q column and a Molsieve 5 column; Ar was
used as carrier gas for GC analysis of H2.
Results and Discussion
Propane dehydrogenation under oxygen-free and oxygen-lean conditions
Operation under the oxygen-lean or oxygen-free conditions described in this paper results in
highly selective propane dehydrogenation. The propene selectivity was above 90% at a
propane conversion of up to 30 % (Fig. 1). The main side products were CH4, C2H4 and
C2H6. However, their individual selectivity did not exceed 5 % even at a propane conversion
of approximately 50 %. Selectivity with respect to COx (mainly CO) depends on O2 partial
pressure. The selectivity can be kept below 5 % by operating with C3H8/O2 ratios higher than
10. Tiny amounts of COx were also formed during the first 1-2 hours of operation under
oxygen-free conditions due to the participation of oxygen of VOx–species in over-oxidising
hydrocarbons.
In comparison to oxidative dehydrogenation under stoichiometric conditions (Fig.2, a), the
dehydrogenation of propane under oxygen-lean conditions (Fig.2, c) is significantly more
selective to propene, and results in a propene yield of up to 40 % at 823-843 K. In our
previous contribution 5 we did not observed significant difference in propene selectivity in
oxidative propane dehydrogenation for highly dispersed VOx clusters supported over Sisupports with different morphology (SiO2, MCM-41, and MCM-48). The same is valid for
vanadia supported over SBF-15 (Fig.2, a). Propene selectivity obtained over VOx(5)/MCM41 and VOx(4)/SBF-15 was very similar at comparable degree of propane conversion under
oxidative
dehydrogenation
under
stoichiometric
conditions
(Fig.2,
a)
and
the
dehydrogenation of propane under oxygen-lean conditions (Fig.2, c).
The differences between previously reported iso-butane dehydrogenation 1 and propane
dehydrogenation under oxygen-lean conditions are the following: 1) due to a stronger C-H
bond in the propane molecule in comparison with that in the iso-butane molecule, 3-4 times
longer contact time is required to achieve a comparable degree of alkane conversion at similar
temperatures 2) thermodynamic limitation of propane dehydrogenation 7 results in a lower
maximal yield of propene at a given T and alkane concentration 3) propane dehydrogenation
is more selective than iso-butane dehydrogenation, in the later case selectivity to side nbutenes reach 15 % at iso-C4H10 conversion above above 50%.
Because
non-oxidative
dehydrogenation
is
significantly
slower
than
oxidative
dehydrogenation (it will be discussed in the next chapter), a significantly longer contact time
(>0.1 s) and slightly higher temperature (T=823-843 K) should be applied to get a significant
contribution of non-oxidative dehydrogenation. However, longer contact times and higher
temperatures are not sufficient to achieve a very high propene selectivity. If reaction is
performed with a stoichiometic inlet ratio of C3H8/O2 =2 (Fig.2, b), the selectivity to propene
will never be higher than 70%, and COx selectivity will be around 30%. The difference
between areas b) and c) in the Fig.2. is the O2 content. To get a really selective performance
in propane dehydrogenation (the same is valid also for iso-butane dehydrogenation) the
reaction should be performed under oxygen-lean conditions, alkane/O2>10. However, we
should emphasize that it is not dependence of selectivity on oxygen content within one
reaction mechanism. With variation of oxygen content the contributions of different reaction
pathways of propene formation, namely non-oxidative and oxidative, are varied.
One more aspect should be commented on at this point. To get a significant contribution of
non-oxidative dehydrogenation, the contact time was increased considerably, by 10-20 times
the original. However, this increased contact time is still in the range of 2-5 seconds.
Supported VOx-aggregates are very active in oxidative dehydrogenation, even when they are
supported on pure SiO2 -supports and an alkane with a relatively strong C-H bond (propane)
is converted. At T =773 K, over supported VOx-aggregates, a high degree of alkane
conversion could be achieved with a contact time of less than 1 second via the oxidative
dehydrogenation route at sufficient O2 content. However, the best results in “oxidative
dehydrogenation” of propane and iso-butane were obtained in the literature mostly at a
contact time of higher than 2 seconds and temperatures of T =823-873 K 12, 13. This is in
area b) of Fig.2, where we started also from the stoichiometric ratio of C 3H8/O2=2; however,
in this area a significant contribution of non-oxidative dehydrogenation takes place, and
formation of H2 was detected.
To get a significant contribution of non-oxidative dehydrogenation we need to apply longer
contact times (2-5 seconds) and slightly higher temperatures (823-843 K). At such conditions
the contribution of gas-phase non-catalytic dehydrogenation is possible. However, these gasphase reactions greatly depend on the free volume in a hot zone of the reactor. All our
experiments were performed with a minimized volume in a hot zone of reactor and we tested
the propane dehydrogenation over fused SiO2 at 833 K and a contact time of 5 g s ml-1. The
degree of propane conversion did not exceed 0.3 % without the catalyst. Thus, the
contribution of non-catalytic gas-phase propane conversion is negligible under the conditions
applied, and propene is formed catalytically and very selectively under these conditions.
Propane dehydrogenation under oxygen-lean conditions can be presented as following. In the
front part of the reactor mainly oxidative propane dehydrogenation takes place with formation
of propene, water and carbon oxides (Schema 1). Most of the VO x species are in the highest
oxidation state 5+. At the reactor outlet predominantly non-oxidative dehydrogenation with
the formation of propene and hydrogen occurs. The reduction degree of VOx species strongly
increases along the catalyst bed. As a result, alkane dehydrogenation occurs via different
reaction mechanisms in different zones of the catalyst bed. We would like to emphasize that
such a combination of ODH and DH proceeds not simply by superposition of two processes.
The catalytic process is complex with a series of competing reactions occurring
simultaneously including carbon deposition resulting in catalyst deactivation. Moreover, this
catalyst deactivation is very likely to proceed with different rates in various reactor zones due
to various reduction degrees of VOx species, different partial pressures of oxygen and reaction
products.
Catalyst deactivation under oxygen-lean conditions
One of the main disadvantages of non-oxidative dehydrogenation in comparison to oxidative
dehydrogenation is a fast catalyst deactivation under oxygen-free conditions. VOx-supported
catalysts are already reported in the literature for non-oxidative dehydrogenation. However, to
our knowledge, no stable performance in alkane dehydrogenation has been reported so far.
Thus, in non-oxidative butane dehydrogenation over VOx/Al2O3 catalysts 8-11 under
comparable conditions already after 30 min of operation 7-8-wt. % of carbon deposits were
detected.
Fig. 3 shows the propene yield obtained over the VOx(5)/MCM-41 catalysts (black circles)
over 20 h on-stream. The VOx(5)/MCM-41 material with highly dispersed VOx-species shows
stable time-on-stream performance. Such stable operation is due to the very low formation of
carbon deposits on the catalyst surface. The amount of carbon deposits formed over the
catalyst after 20 hours on-stream was only 0.8 wt.% of the catalyst mass, as was determined
by TPO (temperature-programmed oxidations). The amount of carbon deposits correlates with
the amount of propene produced. The maximum on the TPO profile for MCM-41- and SiO2based catalysts with highly dispersed VOx-species was below 723 K, i.e. lower than the
reaction temperature. Coke formation increases with increase in Ti content in the support (for
Ti/Si=0.2 ca. 1 wt.% of C, and for Ti/Si=1.5-2 wt.% of C is detected after 20 hours onstream), resulting in less stable performance of VOx/SiO2-TiO2 catalysts (Fig. 3, grey
triangles). It is important that such higher coke depositions are obtained over Ti-containing
samples under the same reaction conditions (temperature, propane concentration, contact
time) and even at lower total amount of propene produced (Fig.3). As evidenced from TPO
tests with used catalysts, another carbon-containing species are formed over the VOx/SiO2-
TiO2 catalyst containing the highest titania (Ti/Si=1.5) amount. These species start to be
oxidized at a ca. 50 K higher temperature. We conclude therefore, that Ti increases the total
amount of carbon deposition under oxygen-lean conditions and the amount of strongly
bonded carbon deposits. As a result, the time-on-stream performance is less stable (Fig. 3).
The stable time-on-stream performance achieved over VOx-supported on SiO2-supports is an
important and significant feature of oxygen-lean operation over silica-supported vanadiumbased catalysts compared to the alkane dehydrogenation over VOx/Al2O3 catalysts 8-11.
The influence of the Ti on the reactivity of supported VOx-species in oxidative and nonoxidative propane dehydrogenation was compared at low degrees of propane conversion
(X(C3H8) < 5%) and at similar V-loading. Whereas Ti dramatically increases propane
oxidative dehydrogenation rate 3, under oxygen-free conditions Ti does not increase the rate
of non-oxidative dehydrogenation (Fig.4).
Conclusions
For the first time, we report on a highly selective and stable propane dehydrogenation to
propene over SiO2-supported dispersed VOx species under oxygen-free and oxygen-lean
conditions. Propene selectivity above 80 % are obtained at propane conversions above 45% at
823-843 K. A stable on stream performance over at least 20 hours on stream was achieved
over silica-supported catalysts due to low coke formation. Propene yield exceeded 40 % under
optimized conditions.
Acknowledgements
Support by Deutsche Forschungsgemeinschaft (DFG) within the frame of the competence
network (Sonderforschungsbereich 546) ‘‘Structure, dynamics and reactivity of transition
metal oxide aggregates” is greatly appreciated.
References
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Figure captions
Fig. 1. Selectivity to main products in propane dehydrogenation over VOx(5)/MCM-41 after 3
h on-stream. T=833 K;W/F=0.3-18 g s ml-1, react. feed: C3H8 - 15 vol. %; O2 - 0 –0.5 vol. in
Ne/N2.
Fig.2. Selectivity to propene vs. propane conversion at different modes of operation over
VOx(5)/MCM-41 () and VOx(4)/SBF-15 () catalysts: a) oxidative dehydrogenation (ODH)
under stoishiometric inlet concentration (C(C3H8)/C(O2) = 2, T=773 K, W/F=0.01-0.04 s g
ml-1, b) oxidative dehydrogenation and non-oxidative dehydrogenation (ODH+DH) under
stoishiometric inlet concentration (C(C3H8)/C(O2) = 2, T=833 K, W/F=0.03-11 s g ml-1, c)
oxidative dehydrogenation (ODH+DH) under oxygen-lean conditions (C(C3H8)/C(O2) > 10,
T=833 K, W/F=0.03-18 s g ml-1.
Fig.3. Propene yield as a function of time-on-stream (TOS) at 843 K over VOx(5)/MCM-41,
VOx(4)/MCM-41 (Ti/Si=1.5) at W/F=11 g s ml-1; reaction feed: C3H8 - 15 vol. %; O2 - 0 –0.5
vol.% in Ne/N2.
Fig.4.
Turnover frequencies of propane conversion over VOx(4-5)/MCM-41 (Ti/Si=0-1.5) in the
oxidative and non-oxidative propane dehydrogenation at 773 and 823 K, respectively.
Fig. 1
100
Selectivity / %
80
C3H6
60
40
15
CH4
C2H6
C2H4
10
5
0
0
10
20
30
40
X (C3H8) / %
50
Fig.2 (can be used as graphical abstract)
100
c)
ODH + DH
C3H8/O2 > 10
S(C3H6) / %
80
60
a)
40
b)
ODH
C3H8/O2=2
ODH + DH
C3H8/O2=2
20
0
10
20
30
40
X(C3H8) / %
50
60
Schema 1
CnH2n+2 + O2
Over oxidized
VOReoxidation
x
O2
CnH2n+2
Oxidized
VOx
Reduced
VOx
CnH2n +H2O+COx
Over reduced
CnH2n+2
CnH2n +H2
VOx
CnH2n + H2 + COx + H2O
Fig. 3.
Yield of C3H6 / %
50
VOx(5)/MCM-41
40
30
VOx(4)/MCM-41 (Ti/Si=1.5)
20
10
0
0
2
4
6
8
10
16
TOS / hours
18
20
Fig. 4
0.1
TOF / s
-1
ODH
0.01
DH
1E-3
0
1
Ti/Si
2