Iodine-Catalyzed Selective Functionalization of Ethane in Oleum: Toward a Direct Process for the Production of Ethylene Glycol from Shale Gas

Article abstract of DOI:10.1021/jacs.0c08975

Direct valorization of ethane, a substantial component of shale gas deposits, at mild conditions remains a significant challenge, both from an industrial and an academic point of view. Herein, we report iodine as an efficient and selective catalyst for the functionalization of ethane in oleum at low temperatures and pressures. A thorough study of relevant reaction parameters revealed iodine to be remarkably more active than the previously reported Periana/Catalytica catalyst under optimized conditions. As a result of a fundamentally different catalytic cycle, iodine yields the bis-bisulfate ester of ethylene glycol (HO3SO-CH2-CH2-OSO3H, EBS), whereas for state-of-the-art platinum-based catalysts ethionic acid (HO3S-CH2-CH2-OSO3H, ETA) is obtained as the main product. Our findings open up an attractive route for the direct conversion of ethane toward ethylene glycol.

Full text of DOI:10.1021/jacs.0c08975

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Article
Iodine-Catalyzed Selective Functionalization of Ethane in Oleum:
Toward a Direct Process for the Production of Ethylene Glycol from
Shale Gas
Marius Bilke, Tobias Zimmermann, and Ferdi Schuth*
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ABSTRACT: Direct valorization of ethane, a substantial compo-
nent of shale gas deposits, at mild conditions remains a significant
challenge, both from an industrial and an academic point of view.
Herein, we report iodine as an efficient and selective catalyst for the
functionalization of ethane in oleum at low temperatures and
pressures. A thorough study of relevant reaction parameters revealed
iodine to be remarkably more active than the previously reported
“Periana/Catalytica” catalyst under optimized conditions. As a result
of a fundamentally different catalytic cycle, iodine yields the bis-
bisulfate ester of ethylene glycol (HO3SO−CH2−CH2−OSO3H,
EBS), whereas for state-of-the-art platinum-based catalysts ethionic
acid (HO3S-CH2−CH2−OSO3H, ETA) is obtained as the main
product. Our findings open up an attractive route for the direct
conversion of ethane toward ethylene glycol.
transfer oxidant and is known as an efficient system for the
selective conversion of methane toward methyl bisulfate
(MBS).15−18 Already at an early stage it was demonstrated
that the sulfuric acid system can in principle also be used for
the functionalization of higher alkanes under relatively mild
conditions.19−22 At temperatures as low as 90 °C the
conversion of ethane to ethionic acid (ETA) is observed,
even in the absence of a catalyst.21,23
Periana et al. built on these findings and more recently
described the efficient and selective C−H functionalization of
ethane to ethionic acid (HO3S-CH2−CH2−OSO3H, ETA)
using Pt(bpym)Cl2 at 160 °C in concentrated sulfuric acid (see
Scheme 1).24 Under catalytic conditions, the authors observed
a highly selective C−H activation process with no detectable
amounts of byproducts that could indicate C−C bond
breaking, overoxidation, or decomposition reactions. Contrary
to the expectations, based on the analogy to methane
functionalization with Pt(bpym)Cl2, the reaction does not
provide the bis-bisulfate ester of ethylene glycol (HO3SO−
CH2−CH2−OSO3H, EBS), which was a product of particular
interest. In addition, depending on the reaction conditions,
INTRODUCTION
■
Ethane is one of the least reactive, but at the same time one of
the most abundant, hydrocarbons. It is present in many natural
gas resources, but particularly its significant share in shale gas
resources makes ethane an increasingly attractive feedstock for
the chemical industry.1,2 On a technical scale ethane is almost
exclusively used for the production of ethylene in the highly
endothermic steam cracking process, being expensive in terms
of energy consumption and investment costs.3 Conversion to
oxy-functionalized products in any case requires additional
processing steps (see Scheme 1).
Although numerous strategies have been pursued to directly
oxidize ethane under mild conditions, hardly any of these have
passed the pilot stage yet.4−6 Proposed reaction schemes
typically suffer from severely limited product selectivity or the
dependence on activated oxidants, such as high-valency main
group element compounds or hydrogen peroxide.5,7 Partic-
ularly promising are routes from ethane to vinyl chloride
combining chlorination and oxychlorination reactions, and a
direct partial oxidation process toward acetic acid.8−10 The
latter one has even been commercialized by SABIC; however,
its economic viability compared to the well-established Cativa
process is certainly questionable.
Received: August 20, 2020
Another strategy that has shown promise for low-temper-
ature short-chain alkane functionalization relies on molecular
catalysts with C−H bond activation as the working
principle.11−14 Here, the “Periana/Catalytica” system, that
has attracted great interest, utilizes sulfuric acid or oleum as the
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A

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Scheme 1. Comparison of the Industrial State-of-the-Art Pathway for Ethane Valorization with Previous Work by Periana et al.
and This Study24
Figure 1. (A) Pressure-time curve indicating proceeding ethane conversion to liquid products (conditions: 15 mL of a 5 mM catalyst solution in
20% oleum, 90 °C; the reactions were stopped after 120 min by quenching the reactor in a water bath with stirring, also see Figure S1). (B)
1
Representative H NMR spectrum of a postreaction mixture showing signals for sulfuric acid as well as EBS and ethylene sulfate.
there are always small amounts of ethyl bisulfate (EtOSO3H)
present which could be identified as the initial product. The
authors propose a sequence of reactions with ethylene
occurring as an intermediate. In an organic follow-up reaction,
which also proceeds in the absence of the catalyst, addition of
the anhydridic S−O bond of pyrosulfuric acid to ethylene
selectively yields ETA. “Free” isethionic acid can subsequently
be obtained by hydrolysis under suitable conditions and is the
starting material for the industrial synthesis of taurine and
numerous surfactants.25
oleum.27 Unfortunately, all of the examples described therein
are limited to reactions of methane. According to Periana et al.,
iodine-mediated reactions of “higher alkanes”, which also
comprise the next homologue ethane, in oleum proceeded with
a significantly lower product selectivity than in case of
methane.26 Although no further details were disclosed, it was
concluded that proton catalyzed reactions in fact make higher
alkanes unsuitable substrates in oleum. Accordingly, oleum
concentrations higher than 5% have not been investigated at
all.
Reaction of an equimolar mixture of methane and ethane
showed that ethane exhibits about 100-fold higher reactivity
under the given conditions compared to the lighter
homologue. This gave rise to targeted mechanistic studies, in
which the authors found fundamental differences compared to
methane oxidation with the same catalytic system. Oxidation of
the alkyl species has been claimed to be no longer rate-limiting;
instead the rate is supposed to be dictated by the initial C−H
activation step. For ethane, C−H activation is followed by
rapid β-hydride elimination, uncatalyzed ethylene functional-
ization, and reoxidation of the Pt hydride complex.
In a previous report molecular iodine was described as an
appropriate catalyst for the oxidation of short-chain alkanes to
the corresponding esters in oleum.26 Similar findings triggered
Bjerrum et al. to even file a patent application, claiming the
functionalization of any gaseous aliphatic hydrocarbon in
In essence, the catalytic use of hypervalent iodine for the
selective oxidation of short-chain alkanes in sulfuric acid/
oleum was thus far limited to methane as the substrate. After
thoroughly (re)investigating this catalytic system in a broad
range of reaction conditions, we here report the use of catalytic
amounts of iodine for the selective functionalization of ethane
in oleum. The iodine system surpasses the previously reported
Pt-based system not only in terms of productivity, but
furthermore the selectivity is altered in a way that the easily
hydrolyzable bis-bisulfate ester of ethylene glycol is obtained as
the only product.
RESULTS AND DISCUSSION
■
Iodine-Catalyzed Ethane Functionalization. The re-
quirement of oleum as the reaction medium, if iodine-based
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compounds are supposed to be used catalytically for methane
oxidation, has been revealed already earlier.18,26,28 A higher
oxidizing power as compared to concentrated sulfuric acid is
needed to stabilize and regenerate cationic iodine species,
which are presumed to be catalytically active. Results obtained
by Periana et al., however, suggested oleum to be inappropriate
for the functionalization of alkanes higher than methane, since
undesired side reactions were observed.26
Considering the above, it was really surprising to see the
outcome of an initial test reaction of ethane with a 5 mM
solution of iodine in 20% oleum (i.e., 20 wt % of “free” SO3),
that was conducted at a substantially reduced reaction
temperature of 90 °C to compensate for the increased
reactivity of the substrate: Within the course of 1 h reaction
time the pressure decreased from initially 30 to almost 15 bar−
being a consequence of ethane conversion to liquid products
(see Figure 1A). 1H NMR analysis of the liquid phase revealed
the bis-bisulfate ester of ethylene glycol (EG(OSO3H)2, EBS)
along with ethylene sulfate ((CH2O)2SO2) as the only reaction
products (X ≈ 60%, S > 98%, see Figure 1B and eq 1). As in
almost all experiments of this study, the carbon balance closed
to well above 90%.
For comparison the “Periana/Catalytica” catalyst was
studied under the same reaction conditions. Here, almost no
change in pressure was observed within the same period of
time (Figure 1A). It is notable that, under optimized
conditions, i.e., with a lower oleum concentration of 1.5 wt
%, albeit an increased reaction temperature of 160 °C,
Pt(bpym)Cl2 yielded only about one-fourth of a valuable
product (ETA) compared to iodine (EBS) in 20% oleum at 90
°C.24
Mechanistic Considerations. Differences regarding cata-
lytic activity and product selectivity of platinum- and iodine-
based catalysts for the functionalization of ethane in sulfuric
acid/oleum are related to fundamental differences regarding
the oxidizability of the participating species and the
corresponding reaction mechanisms. For the iodine-mediated
functionalization of methane in oleum, which, however, is
observed at a temperature of at least 180 °C, experimental and
computational studies suggest either I2 or I+ as the
+
catalytically active species.18,26,28 Differences in coloring of
postreaction mixtures, which were observed after ethane
functionalization, also point toward the presence of different
hypervalent iodine species in solution. It is still unclear whether
these observations are simply a consequence of differences
regarding the oxidizing ability of the mixture. Its oxidizing
power, in turn, is directly related to the SO3 concentration that
highly depends on the ethane conversion level.30
One could try to synthesize cationic iodine species to
perform stoichiometric reactions with the starting material
ethane or other putative intermediates in order to study
mechanistic details. However, synthesis of a cationic iodine
species is generally hampered by the fact that such species can
only be stabilized in oleum.31,32 Based on similar previous
studies,18,33 we have nevertheless performed experiments, in
which molecular iodine was replaced with compounds in which
iodine occurs in oxidation states higher than (0) (also see
Scheme 2A,B). Reactions with catalytic amounts of both I2O5
(oxidation state + V), but also KIO4 (oxidation state + VII)
Subsequent hydrolysis of the reaction solution (water added
in 4-fold volumetric excess, 90 °C, 3 h) gives a mixture of
ethylene glycol and the corresponding mono- and bis-bisulfate
esters in a ratio of 1:0.33:0.01 (see Figure S5).29 Control
reactions under the same conditions as mentioned above
showed no ethyl products in the absence of iodine. Contrary to
generalized assumptions from an earlier report, iodine can
indeed act as a highly active catalyst for the selective ethane
functionalization in oleum under well-chosen conditions.
Scheme 2. (A) Transformation of Different Precatalysts to the Putative Catalytically Active Species Ox-I, the Exact Chemical
Nature of Which, However, Remains Unknown; (B) Generalized Catalytic Cycle for the Iodine-Catalyzed Ethane
Functionalization Reaction in Oleum; (C) Reactivity of Different Iodinated C2 Compounds in Oleum; and (D) Proposed
Pathway for the Functionalization of Ethane toward EBS
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under otherwise identical conditions, gave essentially the same
results as those with iodine (deviation <5%). Since it is well-
known that iodide salts (oxidation state − I) are oxidized in
the presence of sulfuric acid anyway, it can be deduced that,
irrespective of the oxidation state of iodine in the precatalyst,
the same active species is formed under reaction conditions.
The exact nature of this active species, however, remains so far
unknown. In situ spectroscopic studies or computational
methods will be required to identify the active species, ideally
under working conditions. Especially at higher iodine
concentrations oxidation might be incomplete, resulting in
the formation of poly iodo cations, the intrinsic catalytic
activity of which is unknown.34,35
Hypervalent iodine species have previously been reported as
efficient stoichiometric oxidants for the selective oxyfunction-
alization of ethane in nonoxidizing trifluoroacetic acid.
However, in that case the monofunctionalized ester appeared
as the predominant product.36,37 Experiment and theory
disfavor a radical mechanism and support a C−H activation
pathway mediated by the hypervalent iodine complex that is
then followed by M-R functionalization. The high selectivity
and the absence of C1 or overoxidized products, along with the
high reproducibility, support a similar mechanism for the
iodine-catalyzed ethane functionalization in oleum that has
been developed within this study.
Interestingly, EtOSO3H, which might be an intermediate
during the sequential formation of EBS, has never been
observed experimentally. While the iodine-catalyzed function-
alization reaction was not followed systematically over time,
monofunctionalized C2 products were not even detected in
experiments at very short reaction time (10 min) or at reduced
reaction temperature (60 °C). In agreement with results
published by Periana et al.,24 initial control reactions revealed
that ethylene, ethanesulfonic acid, as well as the sodium salt of
ethyl bisulfate undergo quantitative conversion toward a
mixture of ETA and carbyl sulfate when contacted with 20
wt % oleum at elevated temperatures. The same behavior could
be observed both in the absence as well as in the presence of
iodine. However, after carefully reconsidering the relevance of
these results for the catalytic process, crucial deviations from
catalytic experiments are identified: Due to practical and safety
concerns, solid or liquid compounds had to be added to the
reactor before closure, i.e., at room temperature. At room
temperature and in the subsequent heating phase, iodine might
only be partially, if at all, transformed into its catalytically
active form. Given the high excess of oleum ([H2SO4+SO3] ≈
20 M) and the putative intermediates added (1 M), compared
to the concentration of the active catalyst (≤5 mM), at lower
temperatures a reaction pathway, which does not involve any
iodine-containing species, might be preferred. Accordingly,
these control reactions might not be representative for catalytic
experiments.
Thus, for ethylene as putative intermediate, the experimental
design was optimized as follows: In order to find a better
balance between the amount of the active iodine species and
the putative intermediate ethylene, we increased the catalyst
concentration to 25 mM, which is already close to the
solubility limit. The liquid mixture was now heated to the
typical reaction temperature of 90 °C, before ethylene was
actually introduced. By dosing small portions (6 × 1 bar
additional pressure) of a diluted ethylene stream (25% in Ar),
its concentration could be kept low. Assuming full solubility
and ideal behavior, the concentration can be approximated to
be in the range of 15 mM, which is well below the catalyst
concentration.
1H NMR spectroscopic characterization of the reaction
solution revealed a mixture of EBS and ETA along with their
corresponding cyclic analoga ethylene sulfate and carbyl
sulfate, respectively, in a ratio of ∼2:1. Although the formation
of ETA could not be suppressed entirely, this result strongly
suggests the role of ethylene as a decisive intermediate in the
catalytic cycle. The presence of ETA is most likely related to
the fact, that even our optimized experimental design might
not fully reflect catalytic conditions. It is reasonable to assume
that the iodine-mediated reaction of ethylene toward EBS is
fast compared to the C−H activation step. This means that the
steady state concentration of ethylene under catalytic
conditions is very small, which can hardly be reproduced in
the control experiments, and thus, there is still an appreciable
extent of the iodine-independent reaction. One can therefore
conclude, that−given the concentration difference of roughly 3
orders of magnitude - the catalytically active iodine species is
substantially more electrophilic than pyrosulfuric acid. The
latter is the major component of oleum and as such the
relevant species for formation of ETA from ethylene, while the
iodine directs the transformation to EBS. The putative
intermediates ethyl bisulfate or ethanesulfonic acid are solid
respectively liquid at ambient conditions, so that they cannot
easily and safely be dosed at high temperatures under the
demanding conditions. Results for their behavior under
conditions closer to the catalytic experiments could thus not
be studied.
Other possible intermediates that one could envision are
iodoethane, 1,2-diiodoethane, as well as iodoethanol. For all of
these putative intermediates we performed additional experi-
ments: Their gentle addition to 20% oleum in the absence of
iodine at room temperature in all cases led to a vigorous
reaction with a sudden heat evolution, in which the previously
colorless solution immediately turned brownish clearly
indicating the formation of iodine which in turn might be
transformed oxidatively. This observation emphasizes, how
challenging it would be to trap such labile species under
catalytic conditions. However, most remarkably, irrespective of
the nature of the iodinated C2 compound, subsequent liquid-
1
phase H NMR spectroscopic characterization revealed EBS
and ethylene sulfate as the exclusive products in all cases (also
see Scheme 2C). Accordingly, it can be deduced that
iodoethane, 1,2-diiodoethane, as well as iodoethanol might
be key species within the catalytic cycle. Their short-livedness
in oleum further underlines that addition of excessive amounts
of potential intermediates might result in misleading
observations. While for the already difunctionalized com-
pounds, the formation of EBS might just be the result of simple
nucleophilic substitution reactions, for iodoethane an addi-
tional C−H functionalization step is required to yield the
desired product. Although there is so far no experimental
proof, it seems reasonable to assume that the elimination of
iodine from iodoethane in oleum together with the
simultaneous heat evolution could potentially lead to the
formation of a catalytically active iodine species in close
proximity, which then triggers the formation of EBS and
ethylene sulfate. This might also be an explanation for the
differences in reactivity compared to ethyl bisulfate or
ethanesulfonic acid.
Based on these findings, we propose a reaction pathway
(also see Scheme 2D) similar to a mechanism that has recently
D
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Figure 2. (A) Conversion (black box) and combined selectivity (red circle) toward EBS and ethylene sulfate as a function of sulfur trioxide
concentration at 90 °C. A total of 15 mL of a 5 mM [I] solution in sulfuric acid/oleum, initial pressure 30 bar, reaction stopped after 2 h. (B) TOF
(black box) in molproduct mol−1
h−1 and combined selectivity (red circle) toward EBS and ethylene sulfate in % for as a function of sulfur
catalyst
trioxide concentration at 90 °C. A total of 15 mL of a 5 mM [I] solution in sulfuric acid/oleum, initial pressure 30 bar, conversion of ethane <30%.
(C) Conversion (black box) and combined selectivity (red circle) toward EBS and ethylene sulfate in % as a function of temperature. A total of 15
mL of a 5 mM [I] solution in 20% oleum, initial pressure 30 bar, reaction stopped after 2h. (D) Rate in molproduct h−1 (black box) and combined
selectivity (red circle) toward EBS and ethylene sulfate in % as a function of catalyst concentration at 90 °C. A total of 15 mL of a catalyst solution
in 20% oleum, conversion of ethane <30%.
been proposed for the stochiometric oxidation of alkanes with
main group elements in a nonoxidizing acidic environment.38
It involves an electrophilic C−H activation step yielding an
iodo-ethyl species which in turn undergoes an elimination
reaction that results in the formation of ethylene. Ethylene
further reacts to the bisulfate ester of iodoethanol, in which the
labile C−I-bond is cleaved upon nucleophilic attack of the
bisulfate anion to give EBS as the final product.
products, already at relatively low oleum concentrations ethane
is cleanly converted to the bis-bisulfate ester of ethylene glycol.
Ethane conversion increases steadily over the whole concen-
tration range and in some cases almost full conversion could be
achieved. Performing the reaction at a reduced reaction
temperature allows the use of this more strongly oxidizing
reaction medium without sacrificing product selectivity.
Selectivity, however, suddenly drops when [H2SO4 + SO3]
reaches values higher than 21 M. Above that level, ETA is
observed as a side product. Furthermore, increasing amounts
of CO2 were detected in the gas-phase, which indicates that
consecutive reactions of the primary oxidation product take
place.
Caution needs to be paid when selectivity values are
compared, since the conversion levels differ quite substantially
here. However, a similar trend is retained, when conversion is
kept at a constant level (Figure 2B). These undesired side
reactions have been confirmed to take place also in the absence
of a catalyst. Since these reactions proceed faster for higher
oleum concentrations, it is reasonable to ascribe this
phenomenon to uncatalyzed reactions or processes that are
promoted by impurities/species leached out from the reactor
wall. Their contribution to the final product distribution can be
estimated to be up to 20% in the case of 65% oleum.
Influence of Reaction Conditions. Starting from the
remarkable outcome of the initial test reaction as described
above, the dependence of the reaction rate on reaction
conditions was studied. As already described earlier,17,18,39−41
varying the concentration of sulfuric acid and sulfur trioxide
does not only influence the physicochemical properties of the
reaction mixture and thus might affect crucial parameters, such
as catalyst and/or substrate solubility. In addition, it has to be
taken into account that SO3/H2SO4 are directly involved in the
reaction and are accordingly to be regarded as reactants.
Periana et al. observed a strong dependence of the rate of the
Pt-catalyzed ethane functionalization on the H2SO4 concen-
tration: Upon lowering the H2SO4 concentration from 98% to
85% the rate asymptotically decreases.24 Similarly, we
investigated in a systematic manner, if a comparable behavior
is observed for the iodine-catalyzed functionalization of ethane,
with an emphasis on higher oleum concentrations. Figure 2A
displays ethane conversion and selectivity toward the desired
products as a function of SO3 concentration. While in
concentrated sulfuric acid we could not find any ethyl
Quantifying the reaction products formed after a given time
does not necessarily help to resolve intrinsic properties of
different catalysts in some cases. In the present catalytic system
this poses a problem, especially when considering the
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decreasing SO3 concentration as the reaction progresses and
the influence on the catalytic performance, which is linked to
the SO3 concentration. Since in some experiments almost
complete ethane conversion was observed, the reduced
substrate concentration is expected to affect the turnover
rate, which is reflected in a flattening of the conversion plot. To
account for this, a modified operation mode was chosen that
had already been described earlier (also see the SI and Figure
S1).17,41,42 In short, the formation of a liquid product from a
gaseous starting material results in a pressure drop, which
allows the reaction progress to be monitored semiquantita-
tively. By terminating the reaction at a similar pressure drop, it
was ensured that in all experiments an approximately identical
ethane conversion level was achieved (typically 20−25%), so
that a limitation of the reaction due to depletion of any of the
reactants could be ruled out.
Figure 2B shows the TOF toward EBS and ethylene sulfate
as a function of sulfur trioxide concentration. The TOF values
increase linearly over the whole concentration range, which can
be related to a positive reaction order with respect to SO3. For
product selectivity a similar drop as in the previous series of
experiments is observed. Selectivity values obtained with this
differential experimental approach are significantly lower for
oleum concentrations >30% compared to experiments in
which ethane conversion was not kept constant. This can be
related to the contribution of undesired side reactions that is
more pronounced for lower SO3 conversion.
For increasing reaction temperatures increasing ethane
conversion is observed (Figure 2C), as expected. However,
at the same time there is a tremendous decline in selectivity,
which is particularly pronounced for temperatures above 120
°C. For example, at 150 °C, which is the highest temperature
investigated within this experimental series, roughly 8 times
more CO2 is detected compared to the experiment at 90 °C. In
this particular case only 50% of the carbon atoms could be
recovered as carbon dioxide or EBS. Although other potential
side products were not identified, a very similar behavior is
known from surfactant synthesis, where oleum is used for the
sulfonation of fatty alcohols.43 Efficient heat removal is
essential to avoid so-called “product darkening”, being a
consequence of undesired decomposition reactions. Thermal
treatment of solutions of independently synthesized EBS in
20% oleum revealed product stability at 90 °C, whereas at a
temperature of 150 °C almost complete decomposition was
observed within 2 h. One possible decomposition product
could be vinylsulfonic acid, which might form oligomeric
species in the presence of radicals.44
The concept of product protection is no longer as effective
as it was proven to be the case for methane oxidation.45 One
may consider ethane as a “donor-substituted methane”. Under
more severe reaction conditions, at which the reaction rate is
further enhanced, the correspondingly more pronounced
nucleophilicity of C2 compounds outweighs the protective
effect of the electron withdrawing bisulfate substituents and
additional reaction pathways become available. This then
translates into consecutive reactions of the primary product. In
other words: Due to the lability of the products under harsher
conditions one has to find a proper balance between high
selectivity and reaction rate.
flattening. This is most probably due to the fact that the
solution is saturated with iodine from this point on. In other
words, the solubility of iodine can be estimated to be 40 mM
which is in the same range as values reported for comparable
systems.18,41 Moreover, depending on the exact catalyst
concentration, different cationic iodonium species might be
present for a given SO3 concentration.34,46 The differences in
color that were observed for postreaction mixtures might hint
at speciation coming into play. In-depth studies, which would
go far beyond the scope of this work, are required to determine
the nature of the active species and their intrinsic catalytic
properties.
Ethane Functionalization in the Presence of Meth-
ane. To examine similarities between the reactions with
methane and ethane, we performed a competition reaction
with a mixture of both hydrocarbons (V(CH4)/V(C2H6) =
9:1), resembling a composition that is typical for shale gas
deposits. At 90 °C in 20% oleum EBS was observed as the
single product without any C1 oxygenates being present at all
after 2 h. This is unexpected when considering previously
reported rates of electrophilic C−H activation reactions of
various hydrocarbons, and it points toward substantial
differences in the underlying reaction pathways.47,48 There
have been concerns about the need for separation of higher
alkanes, particularly ethane, from natural gas streams before
the major component methane is actually functionalized. This
is due to the fact that for methane functionalization typically
more severe conditions are applied, at which ethane would
mostly undergo total oxidation and thus be lost. Due to the
reduced reaction temperature, the iodine-catalyzed ethane
functionalization we report here might thus be considered as a
viable strategy to separate the higher alkane from the methane
stream by simultaneous valorization.
CONCLUSIONS
■
Our investigations disclose molecular iodine as a highly
efficient catalyst for the selective functionalization of ethane
in oleum, contrary to what has earlier been reported in
literature. The bis-bisulfate ester of ethylene glycol (EBS) can
be cleanly generated at temperatures as low as 90 °C with
reaction rates substantially exceeding those obtained with
previously reported Pt-based catalysts. We could show that
hydrolysis of the postreaction mixture mainly yields ethylene
glycol. Thus, our findings might serve as a basis for the
development of a facile two-step process for the production of
ethylene glycol from shale gas, bypassing the energy-intensive
step of ethylene production that is commonly applied on a
technical scale. Future studies should be directed toward
gaining a more profound mechanistic understanding. Here,
spectroscopic investigations to reveal the catalytically active
species, ideally under working conditions, would be of
particular interest to further optimize the catalyst’s perform-
ance.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.0c08975.
In analogy to a similar study of the iodine-catalyzed methane
functionalization, also for ethane a steadily increasing reaction
rate with increasing catalyst concentration is observed (see
Figure 2D).18 Above a certain concentration level the curve is
Experimental details, information on reactor material
and equipment, chemicals, analytical methods and
calculations, detailed description of procedure for
F
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Encyclopedia of Industrial Chemistry; Wiley-VCG Verlag GmbH & Co.
catalytic experiments, and considerations on product
hydrolysis (PDF)
KGaA: 2011.
́
(10) Lin, R.; Amrute, A. P.; Perez-Ramírez, J. Halogen-Mediated
Conversion of Hydrocarbons to Commodities. Chem. Rev. 2017, 117,
4182−4247.
AUTHOR INFORMATION
Corresponding Author
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́
(11) Caballero, A.; Perez, P. J. Methane as raw material in synthetic
Ferdi Schu
̈
th − Max-Planck-Institut für Kohlenforschung,
chemistry: the final frontier. Chem. Soc. Rev. 2013, 42, 8809−8820.
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Catalytic Oxidation of Methane to Methanol - A Critical Assessment.
Angew. Chem., Int. Ed. 2017, 56, 16464−16483.
Mülheim/Ruhr, Germany; Email: schueth@kofo.mpg.de
Authors
Marius Bilke − Max-Planck-Institut für Kohlenforschung,
Mülheim/Ruhr, Germany; orcid.org/0000-0002-0214-
1769
Tobias Zimmermann − Max-Planck-Institut für
Kohlenforschung, Mülheim/Ruhr, Germany
(14) Schwarz, H. Chemistry with methane: concepts rather than
recipes. Angew. Chem., Int. Ed. 2011, 50, 10096−115.
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Wentrcek, P. R.; Voss, G.; Masuda, T. A mercury-catalyzed, high-yield
system for the oxidation of methane to methanol. Science 1993, 259,
340−3.
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.0c08975
(16) Periana, R. A.; Taube, D. J.; Gamble, S.; Taube, H.; Satoh, T.;
Fujii, H. Platinum catalysts for the high-yield oxidation of methane to
a methanol derivative. Science 1998, 280, 560−4.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
(17) Zimmermann, T.; Soorholtz, M.; Bilke, M.; Schuth, F. Selective
̈
Methane Oxidation Catalyzed by Platinum Salts in Oleum at
Turnover Frequencies of Large-Scale Industrial Processes. J. Am.
Chem. Soc. 2016, 138, 12395−400.
Notes
The authors declare the following competing financial
interest(s): A European patent application, dealing with a
process for the conversion of ethane to ethylene glycol based
on the results that are described in this article, has been filed
on December 3rd, 2020.
(18) Gang, X.; Zhu, Y. M.; Birch, H.; Hjuler, H. A.; Bjerrum, N. J.
Iodine as catalyst for the direct oxidation of methane to methyl
sulfates in oleum. Appl. Catal., A 2004, 261, 91−98.
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the reactions of saturated hydrocarbons with PdII, PtII‑IV, HgII, CrVI,
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MnIII, MnIII-RuIII‑IV, NO2 , H2SO4, H2S2O8, and carbonium ions in
ACKNOWLEDGMENTS
sulfuric acid. Theor. Exp. Chem. 1977, 12, 361−368.
■
(20) Labinger, J. A.; Herring, A. M.; Lyon, D. K.; Luinstra, G. A.;
Bercaw, J. E.; Horvath, I. T.; Eller, K. Oxidation of Hydrocarbons by
Aqueous Platinum Salts - Mechanism and Selectivity. Organometallics
1993, 12, 895−905.
The authors gratefully acknowledge financial support from the
Max Planck Society. Help from the fine mechanics department
at our institute is greatly appreciated.
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