Room temperature and atmospheric pressure aqueous partial oxidation of ethane to oxygenates over AuPd catalysts

Article abstract of DOI:10.1039/d0cy01526a

New modes of chemical manufacturing based on small-scale, distributed facilities have been proposed to supplement many existing production operations in the chemical industry, including the synthesis of value-added products from light alkanes. Motivated by this prospect, herein the aqueous partial oxidation of ethane over unsupported AuPd nanoparticle catalysts is investigated, with emphasis on outcomes for reactions occurring at 21 °C and 1 bar ethane. When H2O2 is used as an oxidant, the system generates numerous C2 oxygenates, including ethyl hydroperoxide/ethanol, acetaldehyde, and acetic acid. Ethyl hydroperoxide is found to be the primary product resulting from the direct oxidation of ethane: it is produced with 100% selectivity in batch reactions with short durations and with low initial H2O2 concentrations. At longer times or in more oxidizing conditions, deeper product oxidations expectedly occur. In batch experiments, the maximum observed yield of oxygenates is 7707 μmol gAuPd-1 h-1. Product distributions differ when H2O2 is replaced by H2 and O2 in the headspace. Additionally, to simulate a scenario wherein H2O2 is produced on-site and to study ethane oxidation in steady, low H2O2 concentrations over 50 h, a semi-batch configuration facilitating continuous injection of dilute H2O2 was implemented. These efforts showed that H2O2 can serve as an oxygenate-selective oxidant of ethane when its concentration is kept low during reaction. These and other experimental results, as well as initial computational results using density functional theory, suggest that paths forward for aqueous ethane conversion exist, and systems should be engineered to emphasize product stabilization.

Full text of DOI:10.1039/d0cy01526a

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Room temperature and atmospheric pressure
aqueous partial oxidation of ethane to oxygenates
Cite this: DOI: 10.1039/d0cy01526a
over AuPd catalysts†
*
Yu Lei Wang, ‡ Sadi Gurses, ‡ Noah Felvey
and Coleman X. Kronawitter
New modes of chemical manufacturing based on small-scale, distributed facilities have been proposed to
supplement many existing production operations in the chemical industry, including the synthesis of value-
added products from light alkanes. Motivated by this prospect, herein the aqueous partial oxidation of
ethane over unsupported AuPd nanoparticle catalysts is investigated, with emphasis on outcomes for
reactions occurring at 21 °C and 1 bar ethane. When H2O2 is used as an oxidant, the system generates
numerous C2 oxygenates, including ethyl hydroperoxide/ethanol, acetaldehyde, and acetic acid. Ethyl
hydroperoxide is found to be the primary product resulting from the direct oxidation of ethane: it is
produced with 100% selectivity in batch reactions with short durations and with low initial H2O2
concentrations. At longer times or in more oxidizing conditions, deeper product oxidations expectedly
occur. In batch experiments, the maximum observed yield of oxygenates is 7707 μmol gAuPd h−1. Product
−1
distributions differ when H2O2 is replaced by H2 and O2 in the headspace. Additionally, to simulate a
scenario wherein H2O2 is produced on-site and to study ethane oxidation in steady, low H2O2
concentrations over 50 h, a semi-batch configuration facilitating continuous injection of dilute H2O2 was
implemented. These efforts showed that H2O2 can serve as an oxygenate-selective oxidant of ethane when
its concentration is kept low during reaction. These and other experimental results, as well as initial
computational results using density functional theory, suggest that paths forward for aqueous ethane
conversion exist, and systems should be engineered to emphasize product stabilization.
Received 9th June 2020,
Accepted 14th August 2020
DOI: 10.1039/d0cy01526a
rsc.li/catalysis
The widespread use of small molecule feedstocks would
have parallel disruptive effects on the global chemical
Introduction
The rapid discovery of geographically dispersed sources of
unconventional feedstocks has provided considerable
motivation for the chemical industry to pursue distributed
industry, which currently relies heavily on conventional
petroleum resources. Molecules of interest in this context
include the light alkanes – the primary constituents of
natural gas – whose global abundance and utility have been
made apparent by the recent shale gas revolution,3,4 as well
as CO2, H2O, O2, and N2, the utilization of which is
considered critical for future sustainability in the energy
sector as well as other high-energy-use sectors, including
chemical synthesis.5–8
chemical manufacturing as
a
supplemental mode of
production.1,2 Advances in renewable energy technologies and
the associated reduction in energy costs at remote locations
provide further impetus for development of a distributed
network of chemical production facilities. Given the reduced
scale and the nature of geographically distributed resources,
new technologies that facilitate the catalytic direct
functionalization of small molecules in mild conditions (low
temperature and pressure and with minimal environmental
impact) are expected to be paramount for any significant
adoption of distributed chemical manufacturing schemes.
In order to develop these technologies, it is essential to
explore the behaviors of catalytic systems in relevant mild
conditions
–
especially near room temperature and
atmospheric pressure. Although the optimal operating
conditions for catalysis will vary greatly among relevant
reactions and processes, even for systems that are required to
operate at elevated temperatures, knowledge of the reactivity
and stability of products in the reaction medium at room
temperature is of critical value.
Oxidative functionalization of light alkanes is particularly
relevant in this context. The local generation of products that
exist in the liquid phase at standard temperature and
Department of Chemical Engineering, University of California, Davis, CA 95616,
USA. E-mail: ckrona@ucdavis.edu
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0cy01526a
‡ These authors contributed equally to this work.
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pressure would alleviate distribution and utilization
constraints that result from the transportation of large
volumes of flammable gases from remote sources.9 Although
the aliphatic C–H bonds of light alkanes are strong, a
number of catalysts have been studied for the low-
temperature oxidative functionalization of alkanes to produce
oxygenates including alcohols, aldehydes, and acids.10–12
Ethane is typically the second-most abundant constituent of
natural gas, and although its direct low-temperature partial
oxidation has not received the same level of attention as that
of methane,13 it is a promising feedstock for the distributed
production of oxygenates in mild conditions.9 Here, the
direct partial oxidation of ethane at unsupported colloidal
AuPd nanoparticle catalysts suspended in water is examined.
confirming the formation of an alloy.14 High-angle annular
dark-field scanning transmission electron microscopy
(HAADF-STEM) images reveal that the AuPd crystals possess
multiply twinned lattice fringes, but nanoparticle
agglomeration resulting from dispersion onto the TEM grid
prevents distinguishing the prevalence of icosahedral versus
cuboctahedral structure, which has been true for other
studies (Fig. 1b and c).15,16 The AuPd nanoparticles have a
mean diameter of 4.93 nm with a narrow particle-size
distribution (Fig. 1d). Au 4f and Pd 3d X-ray photoelectron
spectroscopy (XPS) results show that heat treatment of AuPd
results in the formation of oxidized species Pd2+ and Au3+
(Fig. 1e and f), as has been observed previously in AuPd-
based catalysts.17 However, no distinct surface phases were
detected by XRD or energy dispersive X-ray spectroscopy
(EDX) mapping (Fig. S1†), a typical phenomenon reported in
literature.18 Three independent measurements were taken to
determine the compositions of the catalysts: XPS, inductively
coupled plasma atomic emission spectroscopy (ICP-AES) and
EDX analysis (Table S1†). The Au : Pd molar ratio was
determined by all techniques to be nearly 1 : 1.
Results and discussion
Stabilizer-free AuPd (1 : 1 molar ratio) nanoparticles were
prepared via adaptation of standard colloidal synthesis
procedures involving reduction of metal precursors (PdCl2
and HAuCl4), followed by heat treatment (100 °C). Complete
synthesis details are provided in the Experimental methods
section of the ESI.† An X-ray diffractogram (XRD) of the AuPd
catalyst particles (Fig. 1a) indicates a diffraction pattern with
peaks centered at intermediate values between those of
metallic Au (PDF#04-0784) and metallic Pd (PDF#46-1043),
Catalytic activity was tested in a purpose-built reactor with
all wetted components manufactured from chemically
resistant PEEK plastic. In a typical batch experiment, 5 mL
aqueous AuPd colloid (6.6 μmol of metals) was combined
with hydrogen peroxide (H2O2) at room temperature (21 °C)
Fig. 1 (a) XRD pattern of the nanoparticulate AuPd catalysts. Inset: A photograph of the aqueous AuPd colloidal suspension. (b) STEM image of
AuPd catalysts. (c) Magnification of individual AuPd nanoparticles. (d) Size distribution of AuPd nanoparticles. (e and f) XPS spectra of AuPd in Au 4f
and Pd 3d regions.
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with 1 bar (1 atm) of ethane (C2H6). To our best knowledge,
this work represents the first example for aqueous partial
oxidation of ethane to oxygenates occurring at room
temperature and atmospheric pressure (Table S2†). Liquid-
phase products were quantified using 1H NMR through
independent calibration curves generated from chemical
standards (Fig. S2†), and gas-phase products were analyzed
by gas chromatography (GC).
Fig. 2a provides the quantities of oxygenates produced
and of H2O2 consumed at seven different initial H2O2
concentrations ([H2O2]initial). It is shown that [H2O2]initial
influences the product yields and selectivities, as well as the
efficiency of H2O2 utilization in the oxidative C-H
functionalization process. The selectivity to ethyl
hydroperoxide (CH3CH2OOH, EtOOH) was 100% when the
[H2O2]initial was lower than 30 mM. Given this remarkable
result, the batch reaction with 10 mM [H2O2]initial was
repeated seven times to verify that EtOOH was the sole
Fig. 2 provides results of several aqueous batch reactor
studies. The data indicate that AuPd catalyzes the partial
oxidation of ethane to various oxygenates at room
temperature and atmospheric pressure in the presence of
H2O2. To confirm this result, several control experiments
were performed. It was determined that no liquid oxygenates
were observed in the absence of either C2H6 or H2O2,
indicating that H2O2 was necessary to initiate the C2H6
oxidation reaction, similar to observations made in other
reports.10 Additionally, the potential influence of dissolved
metal ions (with equivalent 6.6 μmole dissolved metal) was
investigated; no products were observed in the presence of
Au and Pd ions but in the absence of AuPd (Table S3,† entry
1). In all C2H6 oxidation experiments, analysis of the
headspace by GC revealed no CO2 was present; however the
product in these conditions. At higher [H2O2]initial
,
acetaldehyde (CH3CHO) and acetic acid (CH3COOH) were
observed (Fig. 2a). Additionally, it was observed that the
−1
maximum amount of liquid oxygenates (7707 μmol gAuPd
h−1) was obtained for 200 mM [H2O2]initial (Fig. 2a and S3†).
The efficiency of H2O2 utilization was also quantified. The
gain factor (defined as mol oxygenates/mol H2O2
consumed)10 was highest for low-[H2O2]initial reactions (Table
S4,† entries 1–7). This phenomenon has been previously
observed10 and can be attributed to the fact that H2O2
adsorption and decomposition competes with C2H6
adsorption for surface sites.19,20 When the reaction whose
results are reported in Fig. 2c was studied with 10 bar C2H6
and the same [H2O2]initial, a much greater quantity of EtOOH
was formed (Fig. S4†), which is consistent with the existence
of a competition for reactant adsorption.
relatively high solubility of CO2 in water19 prevents
a
definitive claim that no complete oxidation occurs (see
discussion later in this report for evidence of acetic acid
oxidation to CO2 in alternate conditions used for mechanistic
studies).
In the reactions above, the decomposition products of
H2O2 (ref. 21) are the active oxidants of the dissolved alkanes;
Fig. 2 Catalytic activity of unsupported AuPd nanoparticles for C2H6 oxidation. (a) Quantities of oxygenates produced and of H2O2 consumed for
seven initial quantities of H2O2, corresponding to 0, 1, 10, 30, 100, 200, and 500 mM [H2O2]initial, respectively. The right-hand side shows results
generated in the absence of H2O2 but in the presence of a H2/O2 mixture. Reaction conditions: 5 mL; 1 mg AuPd; 21 °C; 1 h; 1000 rpm; 1 bar C2H6
or 2.4 bar gas mixture (4.17% H2, 16.7% O2, 37.5% N2 and 41.6% C2H6). (b) Selectivities of EtOOH and CH3COOH for reactions in (a). (c) Quantities
of oxygenates produced and of H2O2 consumed for multiple reaction times. Reaction conditions: 5 mL; 1 mg AuPd; 10 mM [H2O2]initial (50 μmol); 1
bar C2H6; 21 °C; 1–8 h; 1000 rpm. (d) Selectivities to EtOOH and CH3COOH for reactions in (c).
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the results above suggest the existence of competition and
and gain factor correlated positively with reaction time (Table
S4,† entries 3, 8–10). With increasing reaction time, CH3CHO
and CH3COOH were observed (Fig. 2c), consistent with
sequential oxidation of EtOOH to CH3CHO and then
CH3COOH as proposed by Hutchings and coworkers.25
Notably, EtOH was present at detected level at t = 8 h. Further
C2H6 oxidation studies were performed at elevated
temperature (50 °C) with multiple initial H2O2
concentrations. It was found that yields of EtOOH, CH3CHO,
and CH3COOH increased with increasing [H2O2]initial from
100 mM to 200 mM, while the yield of EtOH decreased
slightly (Table S5†), indicating that increased initial
quantities of H2O2 does not facilitate direct EtOH production.
This is consistent with the results of the 21 °C reactions
above (Fig. 2a), which indicated that no EtOH was observed
by increasing the initial amount of H2O2. To further explore
the origin of EtOH observed in reactions occurring at 50 °C,
product solution from a representative 50 °C reaction (with
quantified amounts of EtOOH and EtOH) was stored in an
NMR tube, in the absence of AuPd catalysts and of H2O2, and
the products were analyzed after 6 days and after 12 days. As
shown in Fig. S5,† EtOOH was found to spontaneously
decompose to EtOH in time. These 50 °C reaction studies
lead to the tentative conclusion that EtOH is first derived via
EtOOH decomposition rather than C2H6 oxidation, and that
increased temperature promotes EtOOH decomposition to
EtOH. It is also possible that EtOOH can be reduced to EtOH
by H* generated by H2O2 decomposition.24 These studies
cooperativity
between
C2H6
oxidation
and
H2O2
decomposition. H2O2 is itself a valuable commodity chemical –
it is desirable to generate this compound or its reactive
fragments in situ from O2 and H2. To explore the efficacy of
this approach to the partial oxidation of ethane at unsupported
AuPd nanoparticles, ethane was co-fed with O2 and H2 to the
reactor in the absence of H2O2. The righthand sides of
Fig. 2a and b show the results of these experiments. It was
observed that the distribution of products differs considerably
from that obtained through external H2O2. Specifically, a much
higher ratio of CH3COOH to EtOOH was obtained through co-
fed H2 and O2. It was also observed that ethanol (CH3CH2OH,
EtOH) comprised a significant fraction of products, which was
not observed through direct oxidation by H2O2.
Direct comparison of C2H6 oxidation rate resulting from
in situ reaction of H2 and O2 and from a finite [H2O2]initial is
difficult because the instantaneous concentration of H2O2
and its reactive fragments cannot be known. However, a
reasonable approximation is possible by recognizing that in
the catalytic synthesis of H2O2 at AuPd (H2 + O2 → H2O2) in
these conditions H2 is the limiting reactant, which has been
established in prior literature.22,23 If all H2 were consumed
and converted transiently to H2O2, the initial partial pressure
of H2 in the experiment corresponds to 87 μmol H2O2 (17.4
mM) generated over the course of the batch reaction. The
total quantity of oxygenates generated through reaction of co-
fed H2 and O2 (ca. 4320 μmol h−1 gcat−1) was 5–10 times
greater than that was observed from reaction with H2O2 (425
indicate that in these conditions
a competition exists
−1
and 934 μmol h−1 gcat for 10 and 30 mM [H2O2]initial). It was
between EtOOH decomposition and EtOOH oxidation, with
preference for EtOOH oxidation to CH3CHO and CH3COOH
at relatively high H2O2 concentrations.
considered that the presence of additional O2 in the in situ
experiments could influence product yields, but it was
determined that this could be neglected: O2 was also present
in all experiments involving finite [H2O2]initial because it is a
primary H2O2 decomposition product, and was quantified
(Table S4†).
To generate a first approximation of operable reaction
pathways, a series of direct oxidation studies were performed
on the observed stable oxygenate products: EtOH, CH3CHO,
and CH3COOH. An initial reactant concentration of 2 mM
was selected because it is roughly the equivalent
concentration of C2H6 in the conditions of the reactor studies
above, as calculated based on known C2H6 solubility data. As
shown in Fig. 3a, in the presence of 10 mM [H2O2]initial, EtOH
was oxidized primarily to CH3CHO and CH3COOH; CH3CHO
was primarily converted to CH3COOH. There is evidence for
C–C bond breaking in these conditions: small amounts of
the C1 oxygenates CH3OH (MeOH) and HCOOH were
observed. C1 products have been previously observed in
aqueous C2H6 oxidation studies, where they were proposed to
originate from methyl radicals resulting from C–C scission of
C2H6 or C2 reaction products.25 No liquid- or gas-phase
products were observed from oxidation of CH3COOH with a 2
mM initial concentration (higher initial concentrations are
considered below). Based on these observations, a high-level
reaction pathway for aqueous C2H6 oxidation at colloidal
AuPd was generated and is shown in Fig. 3b.
Based on these observations, the most reasonable initial
interpretation is that the positive effects on oxygenate yield
and the change in product distributions resulting from co-
fed H2 and O2 originate from the presence of molecular H2.
Park et al. reported that molecular H2 can be easily
dissociated into atomic hydrogen (H*) over Pd atoms.24 The
H* could combine with molecular O2 to produce H2O2. Most
likely, intermediates generated during this process activate
C2H6 and promote C2H6 oxidation. The observed EtOH could
originate from direct C2H6 oxidation but also from EtOOH
reduction by H*; this initial report of this catalysis does not
facilitate direct determination of the mechanistic origin of
EtOH in reactions with co-fed H2 and O2. Further dedicated
mechanistic studies are underway to understand the
distinguishing characteristics of catalysis driven by co-fed H2
and O2.
The 100% selectivity toward EtOOH observed at low
[H2O2]initial (Fig. 2a and b) motivated further time-online
experiments of the reaction with fixed [H2O2]initial (10 mM)
(Fig. 2c and d). Both the total amount of liquid oxygenates
This study of ethane oxidation in mild conditions is
motivated by the prospect that new modes of distributed
chemical manufacturing can supplement or in some cases
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Toward the goal of examining ethane oxidation in a
distributed chemical manufacturing scenario where aqueous
H2O2 is produced on-site at a continuous rate, additional
experiments were conducted in a semi-batch configuration,
wherein aqueous H2O2 is fed continuously during reaction.
Results from the batch reactor studies shown above
indicate that H2O2 is utilized most efficiently (highest gain
factor) at low [H2O2]initial
. In the conditions of those
experiments, 100% selectivity to EtOOH was observed at
initial H2O2 concentrations up to 10 mM. In the
continuous-feed experiments, H2O2 was injected into the
reactor through a gas-tight syringe pump at a concentration
and rate that would maintain approximately 10 mM H2O2
throughout the experiment (based on the calculated average
consumption rate of H2O2 from titration experiments and
by adjusting the initial solvent volume). It is stressed that
the real-time concentration of H2O2 in the reactor cannot
be measured, and that no claim is made here that the
steady-state concentration is exactly 10 mM. The experiment
is intended to simulate generally the relevant scenario
where a valuable oxidant is fed continuously and at low
concentration.
In
a
typical continuous-feed semi-batch reactor
experiment, dilute aqueous H2O2 solution was injected at a
constant rate into the reactor for 50 h (details of these
experiments are provided in the ESI†). Through this
methodology, the oxidation of C2H6 at 1 bar headspace
pressure was examined over Au, Pd, and AuPd catalysts
(Fig. 4a–c and Table S6†). Under identical reaction conditions,
reactor experiments with Au and Pd yielded 3.82 and 4.63
μmol of liquid oxygenates, respectively, whereas experiments
with AuPd yielded 6.51 μmol oxygenates and therefore the
highest H2O2 gain factor. It was observed that the distribution
of products differed among the reactions with the three
catalysts. Product distributions from C2H6 oxidation over Au
were weighted toward less oxidized species (i.e. EtOOH/EtOH)
whereas those from oxidation over Pd were weighted toward
more oxidized species (i.e. CH3COOH). In contrast, C2H6
oxidation over AuPd with 50 h continuous-H2O2-feed yielded a
product distribution centered around a species resulting from
an intermediate degree of oxidation (i.e. CH3CHO). These
observations are consistent with expectations – it is known
that Pd is associated with strong binding of O-containing
intermediates and Au is associated with comparably weak
interaction with these species. The binding energy of
O-containing species on AuPd surfaces is closer to optimum
for reaction activity. That is, the binding energy exists at a
peak of the volcano curve associated with the reactivity of
these species according to the Sabatier principle.29,30 It is
logical therefore that Pd catalysts were observed to favor rapid
decomposition of H2O2 and facilitate a greater degree of
product oxidation. Use of Au catalysts is not expected to favor
the formation and stable adsorption of *OH and/or *OOH,
preventing high rates of C2H6 and oxygenate activation (here,
* refers to adsorbed species). The higher total yield of
oxygenates over 50 h is consistent with the fact that AuPd is
Fig. 3 (a) Quantities of products and of H2O2 consumed for oxidation
of C2H6, EtOH, CH3CHO, and CH3COOH over AuPd. Reaction
conditions: 1 mg AuPd; 21 °C; 2 h; 50 μmol H2O2 (10 mM [H2O2]initial);
1000 rpm. For C2H6 oxidation 1 bar C2H6 was used and for oxygenate
oxidation 1 bar N2 was used, with initial 10 μmol each of EtOH,
CH3CHO, or CH3COOH (equivalent to 2 mM initial each). (b) Schematic
of pathways deduced from results of reaction studies.
displace specific production operations in the chemical
industry. In this context, H2O2 is also a notable commodity
chemical whose production is a candidate for a transition
to small-scale, distributed operations. H2O2 can be produced
safely and with minimal environmental impact through
electrochemical devices5,26 (with air and water as reactants)
and thermal catalytic microreactors27 (with air and H2 as
reactants). Additionally, the expense of H2O2 production
through the traditional anthraquinone process5 and its
subsequent transportation is expected to place additional
cost burdens on any small-scale alkane conversion facility.
Although source-dependent, the cost of H2O2 is
approximately $0.345 per lb (50% solution), and the cost of
freight is estimated to be $3.50 per mi, regardless of the
volume required for the application.28 On-site production
eliminates freight costs, and the H2O2 itself could be highly
cost-competitive when produced in small-scale distributed
facilities. For example, in a recent breakthrough in the area
of distributed electrochemical H2O2 production, it was
reported26 that continuous streams of electrolyte-free H2O2
solutions, up to 20 wt%, could be produced. In that study,
the cost of H2O2 was estimated to be $0.07–0.15 per lb,
depending on the anodic reaction employed in the
system.26
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Fig. 4 (a–c) Products formed by C2H6 oxidation over AuPd, Au, and Pd catalysts for 50 h using the continuous-H2O2-feed configuration described
in the main text. 1 bar C2H6. (d–f) CH3COOH oxidation in the same system as (a–c). 500 μmol [CH3COOH]initial, 1 bar N2. Reaction conditions:
colloidal AuPd, Au, or Pd present with 6.6 μmol of metal; 21 °C; 50 h reaction time; 1000 rpm, 500 μmol H2O2 total injected over 50 h period.
associated with an optimal *OH and/or *OOH binding energy
for this reaction.
conditions associated with the continuous-H2O2-feed
configuration. Fig. 4d–f and Table S6† provide quantified
product distributions associated with this reaction with 100
mM initial CH3COOH concentration (that is, the identical
experiment whose results were reported in Fig. 4a–c for C2H6
oxidation, but with dissolved CH3COOH as the reactant).
In these reaction conditions and in the presence of Au,
Pd, or AuPd catalysts, CO2 was found to be a prominent CH3-
COOH oxidation product in the headspace. This indicates
that when present in sufficient concentrations, CH3COOH
readily undergoes both C–H activation and C–C bond
cleavage. CH3COOH oxidation over AuPd yielded a greater
overall quantity of products compared to Au and Pd, as was
the case for C2H6 oxidation. It is clear from these results that
in this reactor configuration (1 mg AuPd per 5 mL water), the
combination of low temperature, low H2O2 concentration,
and the stabilizing effect of water are insufficient to prevent
overoxidation of oxygenates present in sufficiently high
concentration. Specifically, at room temperature and with
In the conditions of the batch reactor studies above, low
[H2O2]initial resulted in 100% EtOOH-selective C2H6 oxidation
over AuPd at reaction times on the order of 1 h. However, the
H2O2-continuous-feed semi-batch studies indicate that even
at low average steady-state H2O2 concentration, deeper
oxygenate oxidations occur (Fig. 4a). In these experiments,
however, CO2 was only observed in the headspace in
extremely small quantities (Fig. S6†). This result suggested an
attractive scenario could exist, wherein the most oxidized
oxygenate product observed, CH3COOH, could be stable in
these catalytic conditions (room temperature and
atmospheric pressure). Acetic acid, CH3COOH, is an
important compound for
applications.31
a
number of industrial
Given this interest, the oxidation of aqueous CH3COOH in
the presence of Au, Pd, and AuPd was directly investigated at
higher initial concentration and in the milder oxidizing
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100 mM initial concentration, reaction over AuPd yielded a
CH3COOH conversion of 3.25%.
to acetaldehyde, which can be further oxidized to acetic acid.
Ethanol is observed as a product when H2O2 is used as
oxidant, but results indicate it originates from the
decomposition of ethyl hydroperoxide rather than from the
direct product of ethane oxidation. Given these observations,
and motivated by the prospect of distributed manufacturing
of value-added chemicals from alkane feedstocks in mild
conditions, this study also reported results simulating the
utilization of H2O2 produced on-site at continuous rates.
Through use of a pressure-tight semi-batch configuration
with continuous dilute H2O2 feed, it was determined that
H2O2 could be utilized much more efficiently as an
oxygenate-selective oxidant of ethane when low H2O2
concentrations are maintained for the duration of the
reaction. The presented results indicate that aqueous
catalytic ethane oxidation over unsupported AuPd produces a
range of value-added C2 products, but additional efforts are
needed to stabilize these products from further oxidation.
Given this need, a continuous process optimized for product
stabilization could serve as the basis for effective distributed
oxygenate synthesis in mild conditions from ethane.
The product quantities reported in Fig. 4 for C2H6
oxidation and CH3COOH oxidation are not directly
comparable because the concentration of the reactants
differed considerably. Given the general interest in a reaction
system that produces C2 oxygenates from C2H6, preliminary
calculations using density functional theory (DFT) were
performed to determine C-H activation of C2H6 and
CH3COOH. Previous studies have reported that ˙OH or ˙OOH
obtained from the decomposition of H2O2 are the active
species for the initial activation of these molecules.10,25,32,33
Given these precedents, barriers were calculated for
H
abstraction from the molecules by *OH and by *OOH on the
surface of AuPd (Fig. S7 and S8†). The results, which
represent a highly simplified first approximation of this
reaction step, indicate H abstraction by *OH results in a
lower barrier for both C2H6 and CH3COOH. Recognizing that
similar activation energies exist for both molecules, it is
necessary to design reaction systems capable of stabilizing
the oxygenates and preventing overoxidation.13
More generally, these results show that at low
concentrations, acetic acid is relatively unreactive even in this Conflicts of interest
simple aqueous system (Fig. 3a), and experiments are
ongoing to determine the role of water in influencing
There are no conflicts to declare.
reaction outcomes. The presence of water in similar catalytic
systems – where both oxygenates and water bind to active
Acknowledgements
sites through the oxygen atom – is known to effect the
C. K. acknowledges support from American Chemical Society
Petroleum Research Fund 59999-DNI5. The authors
acknowledge Prof. Ambarish Kulkarni for providing the
preliminary DFT calculations. The authors acknowledge
Patrick Beckett from the University of California, Davis for
providing XRD measurements.
removal of adsorbed products, resulting in the accumulation
of stable products in the solution. Encouraging results exist
on this front – it has been reported that up to 0.5 M
CH3COOH produced by C2H6 oxidation can be stabilized in
an aqueous system if H2O2 is produced concurrently (in
tandem) at a slow and steady rate.34
#
Notes and references
Conclusions
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