Oxidative dehydrogenation of ethane on dynamically rearranging supported chloride catalysts

Article abstract of DOI:10.1021/ja505411s

Ethane is oxidatively dehydrogenated with a selectivity up to 95% on catalysts comprising a mixed molten alkali chloride supported on a mildly redox-active Dy₂O₃-doped MgO. The reactive oxyanionic OCl^- species acting as active sites are catalytically formed by oxidation of Cl^- at the MgO surface. Under reaction conditions this site is regenerated by O₂, dissolving first in the alkali chloride melt, and in the second step dissociating and replenishing the oxygen vacancies on MgO. The oxyanion reactively dehydrogenates ethane at the melt-gas phase interface with nearly ideal selectivity. Thus, the reaction is concluded to proceed via two coupled steps following a Mars-van-Krevelen-mechanism at the solid-liquid and gas-liquid interface. The dissociation of O₂ and/or the oxidation of Cl^- at the melt-solid interface is concluded to have the lowest forward rate constants. The compositions of the oxide core and the molten chloride shell control the catalytic activity via the redox potential of the metal oxide and of the OCl^-. Traces of water may be present in the molten chloride under reaction conditions, but the specific impact of this water is not obvious at present. The spatial separation of oxygen and ethane activation sites and the dynamic rearrangement of the surface anions and cations, preventing the exposure of coordinatively unsaturated cations, are concluded to be the origin of the surprisingly high olefin selectivity.

Full text of DOI:10.1021/ja505411s

Article
pubs.acs.org/JACS
Oxidative Dehydrogenation of Ethane on Dynamically Rearranging
Supported Chloride Catalysts
Christian A. Gartner,† Andre C. van Veen,†,‡ and Johannes A. Lercher*,†
́
̈
†Department of Chemistry and Catalysis Research Center, Technische Universitat Munchen, Lichtenbergstraße 4, 85747 Garching,
̈
̈
Germany
‡School of Engineering, The University of Warwick, Library Road - Office A421, Coventry CV4 7AL, U.K.
S
* Supporting Information
ABSTRACT: Ethane is oxidatively dehydrogenated with a selectivity up to 95% on
catalysts comprising a mixed molten alkali chloride supported on a mildly redox-
active Dy2O3-doped MgO. The reactive oxyanionic OCl− species acting as active
sites are catalytically formed by oxidation of Cl− at the MgO surface. Under reaction
conditions this site is regenerated by O2, dissolving first in the alkali chloride melt,
and in the second step dissociating and replenishing the oxygen vacancies on MgO.
The oxyanion reactively dehydrogenates ethane at the melt−gas phase interface with
nearly ideal selectivity. Thus, the reaction is concluded to proceed via two coupled
steps following a Mars-van-Krevelen-mechanism at the solid−liquid and gas−liquid
interface. The dissociation of O2 and/or the oxidation of Cl− at the melt−solid
interface is concluded to have the lowest forward rate constants. The compositions
of the oxide core and the molten chloride shell control the catalytic activity via the
redox potential of the metal oxide and of the OCl−. Traces of water may be present
in the molten chloride under reaction conditions, but the specific impact of this water is not obvious at present. The spatial
separation of oxygen and ethane activation sites and the dynamic rearrangement of the surface anions and cations, preventing the
exposure of coordinatively unsaturated cations, are concluded to be the origin of the surprisingly high olefin selectivity.
A recent review of the main classes of catalysts considered for
ethane ODH indicates that for most catalysts the first C-H
bond activation either via single electron transfer (i.e., radical)
or involving paired electrons (i.e., redox) is rate determining.4
While in nearly all the cases, the initial CH bond is broken
homolytically, catalysts that operate via a Mars-van-Krevelen-
redox mechanism exhibit, in general, high ethene selectivity,
while catalysts supporting free radical pathways yield high
selectivity to COx. The factors governing activity and selectivity
include the metal−oxygen bond strengths in oxide catalysts, the
specific atomic arrangement of the active sites, the concen-
tration of these sites, as well as the ease of desorption and
readsorption of ethene.4
Supported alkali chloride catalysts have been reported to
show a surprisingly high olefin selectivity in the oxidative
dehydrogenation of ethane. The excellent selectivity of such
catalysts has been associated with the fact that the catalysts are
supported alkali or alkaline earth chlorides that are molten and
therefore dynamically rearrange under reaction conditions.5,6
Ethene selectivity in the ODH of ethane has been reported to
exceed 90%. The low density of sites and the facile desorption
of ethene seem to be especially relevant factors. The density of
sites is low, as the active species are concluded to be dissolved
in the melt and homogeneously distributed, making it less likely
INTRODUCTION
■
The oxidative dehydrogenation (ODH) of ethane is concep-
tually a selective route from ethane to ethene, which replaces
the external energy input in the generation of ethene by
coupling it to the oxidation of hydrogen. The economic
feasibility of the route, however, necessitates a very high olefin
selectivity, as it competes as a process with ethane steam
cracking. To be viable, the process must avoid not only the
losses by total oxidation, but also the formation of aromatic
byproducts.1
Oxidative dehydrogenation requires the activation of both
reactants, O2 and ethane. It is usually assumed to occur via the
formation of the reactive species either by redox2 or radical3
chemistry. This is followed by the abstraction of two H atoms
and the formation of a C-C double bond upon desorption. As
the formed π-bond interacts strongly with coordinatively
unsaturated metal cations, it tends to react further on many
redox active catalysts. Total oxidation of ethane and ethene is
also more exothermic than ODH, thus being thermodynami-
cally favored. An ideal catalyst must therefore be selective for
C-H bond breaking but not for oxygen insertion, the reaction
that eventually leads to the total oxidation of the carbon atom
to which the oxygen is attached.4 Such a catalyst should also be
able to adsorb the paraffin more strongly than the olefin.
Ethene desorption must be fast and the readsorption of ethene,
mainly facilitated by coordinatively unsaturated metal cations
on surfaces, should be minimized.
Received: June 8, 2014
© XXXX American Chemical Society
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that ethene formed reacts again with such sites. In addition, the
molten overlayer provides a dynamically rearranging surface,
which is able to minimize the readsorption of ethene, as the
chloride melt termination does not allow a long exposure of
metal cations to the gas phase. Because activity and selectivity
critically depend on the nature and concentration of these
dynamically forming active sites, we decided to explore this
catalytic chemistry mechanistically, and to study the concen-
tration and dynamic genesis of the active sites under reaction
conditions.
For supported alkali chlorides, two different mechanistic
hypotheses have been presented. The first proposes the
transient formation of OCl− as catalytically active species, the
ODH reaction taking place at the surface of the overlayer.6 The
second suggests the formation of the redox pair Li2O/Li2O2 as
active species, which is modified and protected by Cl− anions.7
However, both hypotheses have not been substantiated and
quantified experimentally. It is reported that alkali chlorides are
most effective, if they are used in eutectic compositions,
providing faster access to a supported liquid salt. With LiCl as
one component, the second can be a different alkali or an
alkaline earth chloride.5,6 Decreasing the melting point of the
eutectic compared to pure LiCl results in an increasing
selectivity toward ethene, but also decreasing activity. In this
study, we have chosen Li-K-Cl in eutectic composition as our
model system, since it leads to the catalysts with the highest
olefin selectivity reported so far.6 Additionally, the Li-free
eutectic system (Na-Cs)Cl was studied to probe the necessity
of the presence Li+ cations for selective catalysis, aiming to
develop a comprehensive mechanistic model, which addresses
elementary steps and distinguishes between the mechanistic
hypotheses. This holds especially true for the role of the
support, while various overlayer compositions have already
been studied.6 The activation of O2 and ethane has been
addressed using transient kinetic studies, implemented in step
experiments, which additionally allow the quantification of the
O2 and ethane uptakes. Temperature programmed isotopic
exchange experiments enable the quantification of the oxygen
dissociation of both support and catalyst, providing important
information about the oxygen dissociation step.
● ○
Figure 1. ODH and side reactions: , :Li-K-Cl/MgO + Dy2O3
■ □
(entry 5, Table 3), , : Na-Cs-Cl/MgO + Dy2O3 (entry 10, Table 3);
0.3 g catalyst, WHSV = 0.8 h−1 to 2.0 h−1, PO2 = PEthane = 80 mbar.
which is concluded not to be exposed to the gas phase), the
catalytic activity is concluded to involve Cl.
Impact of the Nature and Thickness of the Chloride
Overlayer. As the chloride anion plays a key role in the ODH
of ethane on supported alkali chloride catalysts, the influence of
its concentration has been quantitatively explored.
A series of catalysts with different molar ratios between the
chloride overlayer (LiCl/KCl in eutectic composition5,6) and
the support (physical mixture of MgO as main support
component and Dy2O3 as an additional component minor
component, i.e., dopant, present as small particles on the
surface of MgO) were tested for their activities and surface
properties. The Supporting Information (SI) (Figure S1)
includes an in situ X-ray diffraction analysis of Li-K-Cl/
MgDyO. It is shown that the oxidic support and the chloride
overlayer remain in different crystalline phases at ambient
temperature and do not form a mixed crystalline phase. Figure
S2a (SI) shows the specific surface areas of the catalyst as
functions of the molar overlayer fractions (Noverlayer/(Noverlayer
support).
+
N
Table 1 reports the theoretical film thicknesses, for the
assumption that the surface area of the support is 69 m2 g−1
RESULTS AND DISCUSSION
Table 1. Estimated Average Chloride Layer Thicknesses
(assumptions: pores are not filled, densities of LiCl and KCl
at 20°C)
■
Role of Cations and Chloride for the Oxidative
Dehydrogenation of Ethane. Figure 1 compiles reaction
data for two different eutectic overlayers (Li-K-Cl, mp = 353
°C, Na-Cs-Cl mp = 486 °C) on MgO+Dy2O3. The rates of
ODH markedly exceeded those of total oxidation on both
catalysts, i.e., the selectivity to ethene varied between 93% and
100% for Li-K-Cl and between 86% and 94% for Na-Cs-Cl. The
pre-exponential factors for the total oxidation were by far lower
than the preexponential factors for ODH, indicating only a very
small concentration of unselective sites. This difference in the
preexponential factors makes the variations in the apparent
activation energy less relevant for the activity and selectivity. It
is interesting to note that the preexponential factor for CO2
formation was higher in the case of Na-Cs-Cl, indicating that
with larger cations (60% Cs+ in the case of Na-Cs-Cl, 42.5% K+
in the case of Li-K-Cl) a higher degree of total oxidation was
observed. Because both catalysts had similar activities, it is
conclusively demonstrated that the redox pair Li2O/Li2O2 is
not essential for ODH of alkanes. In turn, as chloride is the
common component in both catalysts (besides the support,
calculated film
sample
no.
overlayer estimated film thickness for thickness for 33 m2 g−1
[mol%]
68 m2 g−1 [nm]
[nm]
1
2
3
4
5
6
7
1.3
3.4
0.1
0.3
0.6
1.1
2.2
4.4
6.6
0.2
0.6
5.6
1.2
10.7
19.2
32.2
41.7
2.3
4.6
9.2
13.8
(fresh support) and for the assumption that the surface area of
the support is 33 m2 g−1, i.e., the specific surface area after
removal of all chloride layers.
While the pure support had a surface area of 68 m2 g−1, the
addition of the minor overlayer of 1.3 mol % alkali chlorides
decreased it to only 12 m2 g−1. The addition of more chloride
resulted in a further, but slower, decrease to 5 m2 g−1 at a molar
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Figure 2. Activities of catalysts with different support/overlayer molar ratios. (LiKCl/MgO + Dy2O3, T = 625 °C, WHSV = 0.8 h−1, Ptotal = 1 bar,
pEthane = pO2 = 70 mbar; for conversions see Figure S3 of the Si.)
overlayer concentration of 32%. While we assume for these
calculations that pore blocking or pore filling did not occur, we
think that in reality a fraction of pores will be only partly filled
with chloride.
Table S1 of the SI shows the cumulative pore volume of the
different samples. Thus, the real film thickness might be greater
on average than ideally expected but also widely varying as
clearly shown by the transmission electron microscopy graph in
Figure S4 of the SI. Obviously, the film thicknesses measured in
the example by HAADF-TEM are significantly higher than the
calculated film thicknesses compiled in Table 1. In addition to
the heterogeneity, this suggests that the chloride layer must
have induced significant sintering of the support.
To test this hypothesis, a catalyst (composition according to
entry 6 in Table 3) was treated with water in an ultrasonic bath
after calcination to wash off the chloride layer. Subsequent BET
analysis showed a specific surface area of 33 m2 g−1, indicating
chloride-induced sintering of the oxide support. However, as
about half of the specific surface area and the pore volume were
restored, we conclude that part of the pores were blocked but
not filled. The average nominal overlayer thicknesses of the
catalysts (without considering pore blockage) for this specific
surface area of the support are also compiled in Table 1.
Figure 2 presents the formation rates and selectivities of
catalysts with different overlayer molar fractions. The MgO +
Dy2O3 support had modest reactivity and selectivity. This is
consistent with the literature reporting moderate olefin
selectivities in ODH for oxides of alkaline earth and rare
earth metals.3 Poor selectivity of these oxides resulted from
facile formation of alkyl radicals that tend to be easier fully
oxidized either at the oxide surface2 or in the gas phase
subsequent to their desorption.3
Addition of the smallest increment of chloride almost
doubled the ODH selectivity and increased the rate normalized
to the catalyst weight. The addition of more chloride resulted in
a further increase in activity and alkene selectivity, reaching an
asymptotic value of around 93% selectivity. The high activity
and selectivity was achieved once an average full layer of the
alkali chloride was formed. Ethene formation rates tripled upon
the initial addition of chloride, suggesting either a boost of
activity by chloride or the generation of new sites.
overlayer) showed only 30% of the ODH rate compared to
the maximum. The addition of increasing concentrations of
chloride is concluded to cover the support, blocking first pores
of the support, followed by the formation of islands of melt at
the outer surface of the support and eventually covering it
completely. Apparently, very low concentrations of chloride
loaded (see entries 2 and 3 in Table 3) do not prevent the
exposure of unselective sites on the supports. Only if a thicker
continuous layer is established are the unselective sites covered
and the molten overlayer, illustrated by the transmission
electron microscopy graph (Figure S4 of the SI), formed.
The increase (and later) decrease of the catalytic activity as a
function of the loading with chloride is concluded not to be
caused by changes in the support surface (the specific surface
area dropped upon addition of chloride) but by the generation
of new active sites in the presence of the chloride overlayer.
The decrease in activity at higher chloride loadings is attributed
to diffusional constraints. Thus, the chloride overlayer is
hypothesized to generate a new type of site catalyzing the
relatively fast and selective formation of ethene from ethane via
ODH. The decrease of the activity with higher chloride loading
is concluded to be a consequence of the combination of
decreasing specific surface area (after the overlayer has been
established) and the complex reaction mechanism shown
below. The linear decrease of the rate as a function of the
square root of the chloride overlayer thickness suggests that
diffusion limitations of the oxidizing species could be the cause
for the decreasing reaction rate (Figure S5 of the SI). As will be
demonstrated below, the oxidation of Cl− at the surface of the
support has one of the lowest forward rate constants. As the
dissolution of O2 in the molten chloride is quasi-equilibrated
and because the outer surface hardly varies with the thickness of
the overlayer (Figure S2a of the SI), the concentration of
hypochlorite anions in that layer and, hence, on its surface is
concluded to decrease. This decrease together with the lower
specific surface area must lead to a lower rate in turn.
Table 2 summarizes activation energies and pre-exponential
factors of catalysts with different chloride loadings (graphical
determination see Figure S7 of the SI).
The bare support (MgO + Dy2O3) showed similar activation
energies and pre-exponential factors for ODH and side
reactions. The addition of chloride (even in low concen-
trations) resulted in a drastic increase in the activation energies
and pre-exponential factors for both reactions. Activation
energies of ODH were in the same range (approximately 140
A plateau in ethene productivity was observed as the
concentration of active sites increased continuously followed by
a decrease in catalytic activity at higher chloride loadings. The
catalyst with the thickest overlayer (around 40 mol %
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water and analyzing the removed phase by chemical analysis
(ICP-OES). The results proved the quantitative absence of
cations from the support (Mg2+, Dy3+) in the chloride melt and
the stoichiometry of chloride anions and alkali cations. Thus,
we conclude that the chloride overlayer does not change in
composition during catalysis and that oxidic or chloride species
of the support anions do not exist at the surface of the chloride
overlayer. This does not rule out the presence of, for example,
very small equilibrium concentrations MgO support in a LiCl−
KCl eutectic melt at 873 K, which have been measured
Table 2. Activation Energies and Pre-exponential Factors of
ODH and Total Oxidation Reactions of Catalysts with
Different Chloride Concentrations (WHSV = 1.6 h−1, Pges
1 bar, pEthane = pO2 = 70 mbar)
=
ethene formation
overlayer/ mol% EA/kJ mol−1
A/−
COx formation
EA/kJ mol−1
A/−
0
109
142
143
142
134
135
1.5 × 106
2.1 × 108
2.8 × 108
1.9 × 108
5.2 × 107
2.3 × 107
104
201
209
192
186
178
1.7 × 106
5.6
3.9 × 1010
1.2 × 1011
1.3 × 1010
2.00 × 109
1.3 × 109
10.7
19.2
32.3
41.6
9
potentiometrically to be about 10−7.
Full coverage of the catalyst support can be confirmed by
findings of Kumar et al, who showed in situ Raman
measurements of LiKCl/MgO + Dy2O3 at different temper-
atures. Typical peaks of Dy2O3 (382, 471, and 525 cm−1)
completely disappeared at a temperature of 500 °C, indicating
that the whole support is covered by chloride at this
temperature.5
Considering these facts and especially the low solubility of
organic reactants in the chloride melt, we conclude at this point
that the ODH reaction takes place at the gas−alkali chloride
interface and involves Cl-containing species. At least for the
catalysts with higher chloride loading, which show the best
performance, the redox active sites of the support are not
exposed to the gas-phase reactants. A reaction test with only
SiC reported in Figure S6 of the SI, showing that gas phase
reactions contribute less than 3% of the total rate.
kJ mol−1) for all chloride-containing catalysts, slightly
decreasing with high loadings (in agreement with influence of
diffusion of active species in the layer). The very high values of
the apparent activation energies for the total oxidation
(approximately 180−200 kJ mol−1) together with the high
pre-exponential factors suggest that the main reason for the
high selectivity to ethene lies in the energetic blocking of total
oxidation. The consistently higher preexponential factor
suggests in turn that the transition state for total oxidation is
much looser than the transition state for ODH. A recent
contribution by Chin et al. suggests that the high apparent
energies of activation together with the high preexponential
factor point to radical reactions.8
Elementary Steps of the Oxidation Reaction. To follow
the C−H bond and O2 activation, transient step experiments
were performed to quantify the uptakes of both reactants by the
catalyst. The catalyst was flushed with He for 15 s after a 45-
min exposure to an O2-containing He stream for 45 min and
prior to reaction with 10% ethane in He. Figure 3a presents the
gas concentrations in the effluent.
The low solubility of organic molecules in the chloride melt
makes it unlikely that ethane diffuses through the chloride layer
to the redox active support surface to react, because such low
solubility would lead to severe diffusion limitations under
practical conditions. Thus, the question arises at this point
whether a part of the redox active support is dissolved in the
chloride melt and acts as a distinct redox site at the liquid−gas
interface. The presence of dissolved support in the melt has
been probed by washing the chloride from the catalyst with
After equilibrating the catalyst with O2, ethene formation
persisted for approximately 20 min (Figure 3a), showing that
Figure 3. Step experiment at 625 °C on LiKCl/MgO + Dy2O3. (Catalyst according to entry 5 in Table 3.) (a) Step from 10% O2 to 10% C2H6. (b)
Step from 10% C2H6 to 10% O2.
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the ODH active oxidizing species formed by contact with O2
were retained in the melt. The persistence of the ethene
formation without O2 feed thus eliminates the possibility that
the radical gas phase or surface radical reaction are the
catalytically active species, because such radical intermediates
tend to be quenched rapidly8 and would not be expected to be
stable after 1200 s. Remarkably, CO or CO2 were not observed
in this step experiment, indicating that the reactive intermediate
is ideally selective for the ethene formation and does not
catalyze total oxidation of ethene or ethane. A similar
experiment was performed, feeding the reactants in reverse
order (Figure 3b). Ethene was not detected, demonstrating that
the catalyst did not retain ethane. This can be rationalized by
the high polarity of the melt causing a low affinity for nonpolar
alkanes. Therefore, we conclude that the conversion of ethane
to ethene occurs at the surface of the melt, although the O2
activation and reaction with Cl− occurs in the bulk phase of the
overlayer or on the interface between melt and the molten
overlayer.
An experiment starting under steady state conditions, but with
an abrupt stop of O2 feed (see SI Figure S8) shows that a lower
concentration of active intermediate was present in the molten
phase under steady state conditions. In order to approximate
the rates of the formation of the O−Cl oxidizing intermediate, a
step experiment with a short O2 exposure (1 min) was
performed at various temperatures. The results are compiled in
Figure S9 of the SI.
The activation energy of the O2 activation step is higher than
the activation energy for the overall ODH. Therefore, we
conclude that O2 is activated in an endothermic step and that
the active oxygen oxidizes Cl− in an exothermic step, forming
the active oxidizing species for the ODH, i.e., most likely OCl−.
The stability of the O−Cl intermediate was probed by
additional step experiments with a variable inert purge period
between O2 and ethane exposures. The results are presented in
Figure 4b. The concentration of activated O decreased with
increasing He purge duration. The decay of the stored O can be
described with a second-order kinetics. This indicates that the
depletion process follows a bimolecular recombination, and in
turn this suggests that the disproportionation has been the rate-
determining step, for which the apparent energy of activation of
216 kJ/mol has been determined (Figure S9 of the SI). O2 was
not be detected via MS during the inert purge period. We
conclude that the release rate was too small to be able to detect
the traces of O2 released.
More information on the O2 activation was obtained from
step experiments at various temperatures. The O2 uptake
increased with temperature as shown in Figure 4a.
The increase of the concentration of O2, which has reacted to
form the oxidizing species, with increasing temperature, shows
that O2 uptake is an activated process, likely involving an
endothermic step in the reactive dissociation of O2.
Reaction Pathways of the ODH and Side Reactions.
Having established that it is possible to generate an oxidizing
species (e.g., the hypochlorite anion) that can be stored and
reduced, we used the changes in the concentration of isotopes
of the reactants (steady state transient kinetic analysis
(SSTIKA)) to estimate the rate constants of the individual
steps and the evolution of the products ethene and CO2.
Krypton was added as a tracer to monitor the exact signal of the
step switch between 16O2 and 18O2 during steady state ODH
reaction with unchanged ethane conversion. Figure 5 shows
the responses to the O2 switch on the product CO2 and H2O
and on O2 per se.
The values reported in Figure 4a were the maximum capacity
of O2 uptake in the form of the catalytically active intermediate.
By varying the reaction temperature, the overall concen-
tration of the reactive intermediates and conversion rates of
oxygen-containing intermediates varied drastically. As expected,
the concentration of O2 decreased with increasing temperature
because of increasing O2 conversion levels of the gas mixture,
but eventually the concentrations of 16O2 and 18O2 were nearly
identical at steady-state conditions before and after the step. In
contrast, the concentrations of H2O and CO2 increased with
reaction temperature in accordance with the higher reaction
rates at higher temperatures. While the formation of 16O
containing CO2 disappeared eventually, H216O remained to be
significant even after long time on stream, in concentration
levels that were proportional to the reaction rates of ODH.
Thus, we conclude that the chloride melt must contain a certain
concentration of water under steady state conditions. While
present, that concentration cannot be quantified. After
replacing 16O2 by 18O2 in a step function, unconverted 16O2
disappeared quickly, replaced by 18O2, which reached the
steady-state concentration asymptotically in a timespan that
was shorter for higher temperatures. After the switching,
16O18O reached a maximum soon after the step, slowly
decreasing thereafter (even at 650 °C it took 1000 s until the
last trace of 16O had disappeared). As long as this species is
produced, both oxygen isotopes are present in an exchangeable
form at the solid−liquid interface.
Figure 4. (a) Dependence of O2 uptake on temperature (oxygen
loading time: 45 min; purge time: 0.25 min; (b) Dependence of stored
oxygen on purge time (T = 625 °C). (Catalyst according to entry 5 in
Table 3.)
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Figure 5. SSITKA results at 550, 625, and 650 °C
The behavior of water mainly produced by the ODH
reaction was different. At the lowest temperature, the exchange
between H216O and H218O was very slow. After 60 min H216O
was still the dominant species. At 625 and 650 °C, the
transition from unlabeled to labeled H2O was faster, H218O
becoming the main product after 15 min. There was an
induction period of about 5 min at 650 °C during which only
H216O was formed. This implies that only 16O reactive species
were at the surface of the melt and available for reaction with
the ethane. It could suggest that the formation of the oxidizing
species is slow and competes with O2 exchange and/or that
diffusion of the oxidizing species from the support−liquid
interface to the surface of the melt is significant and may
influence the overall reaction rate.
Also for CO2 a similar behavior was observed, including a
maximum in the rate of formation of the isotopically mixed
product. The time on stream until the last trace of 16O
containing CO2 was detected had been by far longer than the
time 16O18O was observed. This points either to a retention of
CO2 on the catalyst or to differences in the reactivities along
the pathways to form O2 and CO2.10
Experiments To Elucidate the Critical Individual
Reaction Steps. Experiments with C2D6. The activation of
the first ethane C−H bond is the rate-determining step for
most catalytic oxidations11 and for most ODH catalysts (e.g.,
vanadium oxide-based catalysts12). We compared ODH
reaction rates for hydrogenated and perdeuterated ethane to
quantify the kinetic isotope effect (KIE) for LiKCl/MgO +
Dy2O3 (Figure 6a). Ethene formation rates vary in dependence
on the feed (reflected in the pre-exponential factors). However,
the difference is low compared to other ODH catalysts, which
show a kinetic isotope effect.12 Figure 6b presents the
experimentally determined ratio of the ethene formation rates
of undeuterated and perdeuterated ethane compared to the
ratio of rates for a scenario in which the C−H bond activation
would be rate determining. The ratio varies with temperature
but remained well below the values for a KIE in which C−H
bond activation is rate determining. Thus, we conclude that the
cleavage of the first C−H bond does not determine the overall
rate with the supported chloride catalysts.
Figure 6. Catalyzed conversion of deuterated and undeuterated
ethane. (a) Comparison between ethene productivities for C2H6 ( )
■
●
and C2D6 ( ) as reactants; (b) experimental ratio of C2H4 and C2D4
□
production rates ( ), calculated value for the case of a kinetic isotope
○
effect ( )
hydrogen is still involved in the rate-determining step, for
example in the formation of surface hydroxy or alkoxy species
as intermediate products, as also observed with other ODH
catalysts.13
Because the reaction rate ratios of the undeuterated and
perdeuterated ethane were around 1.2, we conclude that
F
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Knowing that the O2 activation is involved in the rate-
determining step, we would like to probe this further by
comparing supports of different O2 isotope exchange capacity,
as we have concluded that the support is critically participating
in the overall reaction.
Influence of the Support Properties. Having established that
the surface of the molten chloride phase is the location of the
catalytic activation of ethane and that the concentration of
oxidizing species determines the overall rate of reaction, the
extent to which the redox properties of the support influence
this catalytic chemistry is probed. This was done by exploring
two oxide supports (bare MgO and ZnO) having less
pronounced redox properties, while maintaining identical
loadings of LiCl/KCl in the eutectic composition as well as a
constant ratio of the support and the overlayer. To rule out
trivial coverage effects, the specific surface areas of the supports
have been thermally adjusted by sintering MgO (Table S2 of
the SI).
Figure 7 shows the activities of catalysts with different
supports. Despite similar chloride loadings MgO showed a
Figure 8. (a) TPIE experiments with different supports and the
corresponding LiKCl-coated catalysts (1: MgO + Dy2O3; 2: LiKCl/
MgO + Dy2O3, 3: MgO, 4: LiKCl/MgO, 5: ZnO, 6: LiKCl/ZnO) (b)
Arrhenius plot of 4 and 6.
(Figure 7) suggests that for ZnO the concentration of sites is
much lower than for the other two oxides. The addition of Dy
leads to higher rates of oxidative dehydrogenation of ethane
(Figure 8), which is attributed to the higher reducibility of
Dy2O3, which leads in turn to a higher rate of formation of the
reactive chloride intermediate and to a higher oxygen storage
capacity compared to that of MgO.
The activity in the isotope scrambling paralleled the reaction
rates in steady state ODH experiments, for which the ranking of
the catalytic activity of the materials was identical to isotope
scrambling rates reported here.
It should be noted in passing that a corresponding
experiment with a mixture of C2H6 + C2D6 did not lead to
isotopically mixed ethane, demonstrating that the hydrocarbons
do not reversibly dissociate CH bonds at the catalyst surface,
generating a concentration of surface-bound H or D which
would permit kinetically significant scrambling. Feeding only
18O2 to MgO- and ZnO-based catalysts, which were heated to
reaction temperature in He flow without being exposed to 16O2
(Figure S10 of the SI), resulted in the formation of 16O2 and
16O18O for a short of time in the case of MgO-based catalysts,
but only marginally in the case of ZnO. Lattice oxygen of the
support oxide is the only possible source of 16O. Oxygen at the
surface of the MgO core is exchanged, exposing simultaneously
O2 dissociation sites. Zinc oxide, however, shows almost no
oxygen exchange. Thus, the mechanism of O2 dissociation and
exchange, depending on the support nature, critically
determines the catalytic activity. We speculate that the
differences are related to the fact that MgO forms only labile
oxygen vacancies (surface defects),14 while ZnO is easier to
▼
Figure 7. Activities of Li-K-Cl- catalysts with different supports (
:
■
MgDyO (entry 5 in Table 3); : MgO sintered at 900 °C (entry in
●
Table 3); : ZnO (entry 9 in Table 3))
much higher reactivity than ZnO. The chemical properties of
the supports drastically affected the catalytic activities although
the supports are covered with the melt and not exposed to the
gas phase. The variations in the catalytic properties were mostly
related to the differences in the pre-exponential factor, while
the activation energies were quite similar. Because of the
similarity of the apparent energies of activation we assume that
the differences lie in the steady-state concentration of oxidizing
species generated at the support−melt interface. We hypothe-
size that a more redox-active support will generate a larger
concentration of oxidizing species (presumably OCl−).
As the O2 dissociation has been concluded to be involved in
the rate-determining step we probe another side of this reaction
by exploring isotope scrambling of 16O2 and 18O2.
A stream containing both O2 isotopes (16O2 and 18O2) was
passed over the catalyst while the temperature was increased
linearly and the formation of 16O18O was followed. This
allowed the exploration of the temperature dependence of the
O2 dissociation rates for MgO, MgDyO, and ZnO (Figure 8).
As expected, the formation rate of 16O18O increased
exponentially with temperature. The formation of the isotopi-
cally mixed O2 asserts that O2 was dissociated and recombines.
The much higher rate found with the MgO-based catalysts and
of the higher apparent energy of activation of that process
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reduce, generating more stable oxygen vacancies due to lattice
relaxation.15
Reaction Mechanism and Conclusions. Let us combine
now the individual experiments to build a mechanistic
hypothesis for the selective catalytic reaction pathway of the
oxidative conversion of ethane to ethene, considering the
proposed sequence of individual reaction steps (Schemes 1 and
because of the thermodynamic equilibrium of the reaction. As a
consequence, we assume it to be most likely that the oxidizing
species is the hypochlorite anion.
The isotope scrambling experiments show conclusively that
the ODH reaction follows a Mars−van-Krevelen type
mechanism with independent, but kinetically coupled steps
for the activation of O2 and ethane. Oxygen is molecularly
absorbed in the polar chloride melt (Step 1). The temperature
programmed exchange between labeled O2 shows that the
exchange takes place at the support surface and is kinetically
hardly limited by the presence of the supported chloride. On
the basis of the rapid transient (Figure 3) and the temperature
programmed exchange reactions, we conclude that O2 is quasi-
equilibrated in the melt and that its concentration is low. The
reactive dissociation of O2 (Step 2) takes place at the interface
between support and liquid melt.
Scheme 1. Suggested Elementary Steps of the Reaction
□
Pathway ( : active Sites on Support; *: Active Sites on
Surface of Melt)
Dissociated O2 at the interface forms an oxygen−chlorine
intermediate (Steps 3 and 4), which we currently speculate to
be hypochlorite anions. These steps are concluded to be
reversible, because the purge period between O2 loading and
ethane exposure decreased the concentration of the active
species. It should be noted in passing that the decay followed a
second order in the concentration of redox active species and
that Cl species have not been detected in the gas phase. The
second-order kinetics leads us to speculate that the decay is
related to a reaction between two hypochlorite anions, but it is
unclear whether this occurs in the melt or at the support
surface. The reactive intermediate (OCl−) diffuses throughout
the molten chloride and, hence, establishes a concentration at
the surface of the melt proportional to the chemical potential in
the bulk and at the two interfaces (Step 5). As shown by the
transient between ethene and O2 exposure (Figure 3b), ethane
is not absorbed into the polar melt. Thus, ethane activation is
concluded to proceed exclusively at the surface of the melt.
Ethane is hypothesized to adsorb on the surface of the melt on
a hypochlorite anion, resulting in the abstraction and the
formation of a subsequent C2H5 radical species and •O− and
HCl (Step 6). The •C2H5 species reacts with the •O− species
and forms a surface complex (Step 7), which is cleaved into
ethene and a hydroxyl anion (Step 8). Finally, OH− reacts with
HCl forming water (Step 9). As ethane scrambling did not yield
isotopically mixed ethane species, those in the latter two steps
are concluded to be irreversible. The adsorbed hydrocarbon
radical reacts with O−, resulting in the formation of ethene and
water via an oxidized intermediate (Steps 6 and 7). Thus, we
conclude that ethane is activated at the gas-melt interface, and
O2 is activated at the melt-support interface. We attribute the
outstandingly high selectivity to the separation of alkane and O2
activation sites.
The nearly ideal selectivity for ODH of the intermediate
species stored in the melt does not preclude, however,
combustion initiated by gas-phase O2. This is shown by the
step experiments (Figure 3), which demonstrate that the
chemically stored oxygen (in the form of an oxidizing species)
is active generating ethene, but not CO2 or CO. The SSITKA
experiments additionally showed a slower O2 isotopic exchange
in the ODH pathway compared to the total oxidation pathway.
Unlike many other oxidation catalysts (e.g., supported
vanadia12 or molybdena16) the H/D isotope effect strongly
suggests that the cleavage of the first C−H bond is not involved
in the rate-determining step over supported alkali chloride. In
the present case, the cleavage of the O2 bond leading to the
oxidizing species appears to be the slow step. Thus, only when
Scheme 2. Schematic Representation of the Reaction
Sequence in Ethane ODH over Supported Alkali Metal
Chloride Catalysts
2). Given the complexity of the reaction sequence we have not
considered several conceivable reaction routes by excluding
them via circumstantial evidence. We are aware of the presence
of a steady state concentration of water in this chloride melt but
did not consider its influence on the chloride properties in this
contribution in other than the explicit ways discussed below.
The transient formation of Cl• radicals or Cl2 has been
excluded on the basis of the absence of corresponding electron
spin resonance spectroscopy signals and of the absence of even
traces of chlorinated species from the product mixtures. The
formation of ethyl chloride, which eliminates HCl to form
ethene has been ruled out, because of the stability of the
chloride catalyst, and no retention of HCl was observed when
scrubbing the reactor exhaust by a caustic solution. We also
ruled out significant hydrolysis of the supported chloride by the
water formed in the reaction and the formation of HCl through
the reaction
NaCl + H2O ⇌ NaOH + HCl
H
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ethane collides with this oxidizing species at the surface of the
melt is reaction possible.
Thus, in summary, we propose a direct and a reverse Mars−
van Krevelen step20 in which ethane and O2 are converted at
different interfaces, namely the molten overlayer/gas phase and
the molten overlayer/solid support interfaces, respectively. The
active oxygen species is formed at the oxygen vacancies at the
support, e.g., MgO in the present case, and diffuses through the
chloride melt to the surface. Thus, the active oxygen species is
dissolved in a molten phase and exists at the surface only in low
concentrations. The species has an anionic character, but it
should be emphasized that, e.g., the hypochlorite is able to
assume also a radical form. Compared to the highly
electrophilic oxygen formed on other typical ODH catalysts,21
the present species react more slowly, but with less tendency to
initiate total oxidation. Depending on the overlayer thickness,
the concentration of active species varies in different catalysts. A
continuous cover helps in increasing the concentration of sites
active for the oxidative dehydrogenation (the apparent
activation energy actually increases), the reactivity of catalysts
decreases, as with increasing overlayer thickness the steady-
state rate of formation of the oxidizing species at the surface of
the support leads to a lower density in the chloride volume and
on the surface.
The present system of supported chlorides offers an exciting
new route toward olefins, which combines high selectivity and
reasonable ethene production rates. While the positive effect of
chloride for oxidative dehydrogenation had been previously
established22,23 and has even been observed in molten systems
of other halogenides,24 the current contribution shows how the
interplay between the support in the chemistry in the supported
chloride may direct alkane oxidative dehydrogenation to new
levels. We are convinced that a further improvement of the
molecular understanding of this system could lead to
improvements in our current access to ethene as well as to
larger and certainly more complex olefins via oxidative
dehydrogenation.
At this point we would like to analyze the molecular
consequences of the spatial separation of oxygen and ethane
activation and the fact that ethane has no physical access to the
oxygen activation site. Oxygen activation on metal oxide
surfaces proceeds conventionally by a stepwise electron transfer
placing the charge on the oxygen ion.11 In the present case, the
adsorbed surface O2 is reduced by redox active sites acquiring a
partial positive charge, while creating a partially negative charge
with the formed oxygen species. A subsequent formation of the
ionic oxygen species is hindered by the limited availability of
electrons at the redox site being part of an isolating oxide. Low
concentrations of such sites and a high barrier (approximately
200 KJ/mol) make this step kinetically slow. The oxygen
activation in the supported alkali chloride system is assumed to
take place near O vacancies which are likely to form in MgO
surfaces.17 Over the vacancy on MgO, O2 is likely to form
−
while exposed to O2,18,19 known as an unselective oxygen
species (Scheme 3), which would form on bare metal oxide
surfaces.
Scheme 3. Possible Molecular Pathway of Chloride-Assisted
O2 Activation at MgO Oxygen Vacancies
The chloride layer of the catalysts studied prevents the access
of hydrocarbons to such active oxygen. The ubiquitous
presence of Cl− facilitates the formation of an oxidized chlorine
species, presumably OCl− (Scheme 3, where this process is
exemplified for hypochlorite as an active species). Its anionic
nature allows the chlorine−oxygen species to desorb readily
from the lattice of the support (being formally replaced by Cl−)
and to diffuse in the melt. As the electrophilic oxygen species
has reacted with Cl−, the strongly electrophilic character has
been reduced and an anion has been formed. The precise
nature of the active oxygen intermediate remains, however, the
subject of further investigation. The chlorine containing
oxyanion diffuses to the surface of the melt, is stabilized there
in low concentrations as isolated species, and reacts eventually
with ethane. The formed ethene rapidly desorbs from the melt
due to its low affinity to a highly polar and dynamically
rearranging surface.
EXPERIMENTAL SECTION
■
Catalyst Preparation. Catalysts were prepared by wet impregna-
tion of chlorides. The support material (MgO (Aldrich, >99%), Dy2O3
(Aldrich, 99.9%), ZnO (Aldrich, 99.9%), or ZrO2 (Aldrich, 99.5%))
was added to 100 mL deionized water, followed by the chloride salts
forming the overlayer (LiCl (Aldrich,>99.5%)), KCl (Merck,99,5%),
NaCl (Merck, puriss.), CsCl (Aldrich, >99.5%)). The slurry was
stirred at 80 °C for 2 h, followed by the evaporation of water under
reduced pressure. The residue was dried at 120 °C for 12 h and
calcined in synthetic air (100 mL/min) at 650 °C for 12 h. Table 3
compiles the chemical compositions of the catalysts.
Table 3. Compositions of Catalysts
no.
support 1 support 2 overlayer 1 overlayer 2 support 1 [mol] support 2 [mol] overlayer 1 [mol] overlayer 2 [mol] overlayer [mol %]
l
MgO
MgO
MgO
MgO
MgO
MgO
MgO
MgO
ZnO
MgO
Dy2O3
Dy2O3
Dy2O3
Dy2O3
Dy2O3
Dy2O3
Dy2O3
−
LiCl
LiCl
LiCl
LiCl
LiCl
LiCl
LiCl
LiCl
LiCl
NaCl
KCl
KCl
KCl
KCl
KCl
KCl
KCl
KCl
KCl
CsCl
0.165
0.165
0.165
0.165
0.165
0.165
0.165
0.168
0.168
0.165
0.0031
0.0031
0.0031
0.0031
0.0031
0.0031
0.0031
0.00129
0.00299
0.0059
0.012
0.000872
0.00293
0.004
1.27
3.40
2
3
4
5
6
7
8
9
10
5.57
0.008
10.67
19.23
32.23
41.65
19.23
19.23
19.23
0.0240
0.048
0.0159
0.0321
0.048
0.072
−
−
0.0031
0.0240
0.0240
0.014
0.0159
0.0159
0.026
−
Dy2O3
I
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To selectively tailor surface properties of MgO, one catalyst
(composition according to row 8 in Table 3) was synthesized using
MgO sintered at 900 °C for 12 h in synthetic air in a muffle furnace.
Physicochemical Characterization. BET surface areas and pore
size distributions were determined by N2 adsorption−desorption at 77
K using a PMI Automated BET Sorptomatic 1900 series instrument.
Prior to the adsorption, the samples were evacuated at 250 °C for 2 h.
ICP-OES. ICP-OES was performed with a SpectroFlame Typ
FTMOA81A ICP-OES spectrometer from Spectro Analytical Instru-
ments. Samples were suspended in deionized water under ultrasonic
treatment. All samples were filtered before analysis.
mg of SiC. After preheating in He to 450 °C and a short equilibration
in the O2 isotope mixture, the temperature was increased with 5 °C/
min to 650 °C, feeding a gas stream of 10 mL/min with a composition
of 2.5% 18O2, 2,5% 16O2 and 95% He. The temperature was held at
650 °C for 15 min. 18O2 with 97% isotopic enrichment was used. For
detection of the different oxygen species, the following m/z signals
were used: 32 for 16O2, 34 for 16O18O and 36 for 18O2.
For the case of ethane, a gas flow of 9 mL/min He, 0.5 mL/min
C2H6 and 0.5 mL C2D6 was fed. The experiment was started at 450 °C
with a temperature ramp of 10 K/min up to 650 °C. The signals at m/
z 30, 31, 32, 33, 34, 35, and 36 were recorded.
Reactant Gases. For ODH of ethane, the following gases (all
supplied from Westfalen AG) were used: He 3.5, ethane (99.995%),
and O2 (10.1%, diluted in He 3.5). For calibrations, ethene 3.5 was
used. For the isotopic labeling studies, C2D6 and 18O2 (97% isotope
enrichment) were used.
ASSOCIATED CONTENT
■
S
* Supporting Information
Several figures describing reaction kinetics and physicochemical
characterization. This material is available free of charge via the
Internet at http://pubs.acs.org.
Catalytic Tests. Catalytic tests were carried out in a plug flow
reactor consisting of a catalyst fixed-bed in a quartz tube surrounded
by a heat distributing block and heating coils. Most experiments used
300 mg of the catalysts diluted in 700 mg SiC (450−600 μm) to
improve the heat transfer and ensure homogeneous temperature over
the entire catalyst bed. Layers of SiC and quartz wool encased the
catalyst bed to minimize dead volume. Reactant flow rates were
individually adjusted by mass flow controllers (Bronkhorst). The
effluent stream composition was quantified by a Maxum Edition II
Process gas chromatograph (Siemens) equipped with TCD detectors.
Oxygen, CO, and CH4 were separated on a Molesieve 5A column (2
m, 60/80 mesh), and the hydrocarbons other than CH4 were
separated on a HayeSep Q column (2 m, 80/100 mesh) combined
with a HayeSep T precolumn (0.5 m, 80/100 mesh). The time-
resolved product stream analysis in transient experiments and isotopic
studies was accomplished with a calibrated Pfeiffer Omni StarTM GSD
320 °C mass spectrometer system connected in parallel.
Steady-State Kinetic Measurements. Standard conditions for
steady-state experiments were partial pressures of ethane (99.5%) and
O2 (10.1% in He) of 70 mbar each with balance He (99.99%) to
atmospheric pressure. A temperature range between 450 and 650 °C
was explored at a WHSV of 0.8 h−1.
Sequential Step Transient Experiments. The catalyst was
heated to the reaction temperature in He and then 10% O2 in He was
subsequently supplied for a variable time (45 min to reach equilibrium,
1 min to establish the kinetics of the intermediate formation in a
different experiment). The probe reaction with ethane (10% C2H6 in
He) followed a purge with 40 mL/min He of variable duration
detecting the ethane consumption and ethene formation by MS
analysis.
AUTHOR INFORMATION
■
Corresponding Author
Johannes.lercher@ch.tum.de
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Funding of this research by SOLVAY as well as fruitful
exchanges with Dr. Michel Strebelle, Dr. Armin Liebens, Dr.
Marco Piccinini, and Paul Degraeve are acknowledged. We
thank Pinghong Xu and Prof. Nigel Browning (UC Davis,
PNNL) for HAADF-TEM measurements. We also thank Prof.
Andreas Jentys and Maximilian W. Hahn, Technische
Universitat Munchen, and Prof. Angeliki Lemonidou, Aristo-
̈
̈
teles University of Thessaloniki, for fruitful discussions.
Christian Gartner thanks the TUM Graduate School and the
̈
Department Graduate Center Chemistry.
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