Direct electrochemical synthesis of oxygenates from ethane using phosphate-based electrolysis cells

Article abstract of DOI:10.1039/d0cc05111j

Ethane was converted directly to acetaldehyde and ethanol by partial oxidation at 220 °C and ambient pressure using an electrolysis cell with a proton-conducting electrolyte, CsH₂PO₄/SiP₂O₇, and Pt/C electrodes. The ethane conversion and the selectivity to the products increased with the voltage applied to the cell. It was found that O species generated by water electrolysis functioned as a favorable oxidant for partial oxidation of ethane on the Pt/C anode at intermediate temperatures. The production rates of acetaldehyde and ethanol recorded in this study were significantly higher than those in preceding reports.

Full text of DOI:10.1039/d0cc05111j

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DOI: 10.1039/D0CC05111J
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Direct Electrochemical Synthesis of Oxygenates from Ethane using
Phosphate-based Electrolysis Cells
Yusuke Hondaa, Naoya Fujiwaraa, Shohei Tadab, Yasukazu Kobayashic, Shigeo Ted Oyamade and
Received 00th January 20xx,
Accepted 00th January 20xx
Ryuji Kikuchi*a
DOI: 10.1039/x0xx00000x
Ethane was converted directly to acetaldehyde and ethanol by selective synthesis of the oxygenates. Several researchers have
partial oxidation at 220oC and ambient pressure using an tried thermal catalytic10-13 or electrochemical14,15 approaches to
electrolysis cell with
a
proton-conducting electrolyte, generate the transient oxidizing species at various
CsH2PO4/SiP2O7, and Pt/C electrodes. The ethane conversion and temperatures. The advantages of the electrochemical approach
the selectivity to the products increased with the voltage applied to are the controllability of reaction rates by changing the rate of
the cell. It was found that O species generated by water electrolysis O2- or H+ ion pumping and the availability of inexpensive
functioned as a favorable oxidant for partial oxidation of ethane on oxidants (O2,15 CO2,16 H2O). In the 1990’s, Frusteri et al.14 and
the Pt/C anode at an intermediate temperature. The production Hamakawa et al.15 conducted the ethane oxidation employing
rates of acetaldehyde and ethanol recorded in this study were electrochemically-generated O species at around 80oC and
significantly higher than those in the precedent reports.
500oC, respectively. They successfully synthesized the
oxygenates by using the electrochemically-generated O species.
However, the reaction temperatures greatly influenced the
selectivity to the oxygenates and the ethane conversion: low
temperature operation (< 100oC) was insufficient to activate
ethane while high temperature (> 400oC) promoted the over-
oxidation of the oxygenates. Accordingly, the increase in the
production rates of the oxygenates is still a challenging issue.
In this work, we expect that the electrochemical reaction at
intermediate temperatures (100-400oC) can attain high
production rates of the oxygenates by enhancing both the
ethane activation and the suppression of the over-oxidation.
However, the electrochemical ethane oxidation at the
intermediate temperatures has not been tested before because
there are few appropriate materials for electrolysis cells. Here a
composite of CsH2PO4 and SiP2O7 was used as an electrolyte
material. The composite has excellent proton conductivity at
around 200oC under humidified conditions.17 Moreover, the use
of oxidizing species (O* and OH*) generated by water
electrolysis was targeted for partial oxidation of ethane in the
anode (Figs. 1a and 1b). A commercial Pt/C sheet was employed
as an anode because Pt metal has catalytic activity for
dissociation of C-H bonds in ethane.2 Regarding the utilization
of the phosphate-based electrolyte at intermediate
temperatures, most studies have demonstrated its application
to fuel cells18-20 and very few studies have demonstrated the
application to electrolysis cells for synthesizing variable
chemicals such as ammonia.21,22 No research has been reported
on the electrochemical conversion of ethane to oxygenates at
Introduction
Natural gas from shale has received much attention as a low-
cost and abundant energy source. Natural gas mainly consists
of methane but also contains substantial amounts of ethane.
The number of the studies on effective ethane utilization has
increased steeply in recent years. Most studies have focused on
ethylene production1-6 while few have addressed oxygenates
production such as acetaldehyde and ethanol.7-9 These products
are difficult to obtain directly from ethane because ethane
activation and suppression of over-oxidation of the oxygenates
are challenging. To overcome these barriers, considerable
efforts have been made so far focusing on oxidizing species
generated in reaction systems. Prior research generally
confirms that transient intermediate oxidizing species such as
O* and OH* are more effective than molecular oxygen for the
a. Department of Chemical System Engineering, The University of Tokyo, 7-3-1,
Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
E-mail: rkikuchi@chemsys.t.u-tokyo.ac.jp.
b. Department of Materials Sciences and Engineering, Ibaraki University, Ibaraki
316-8511, Japan
c. Interdisciplinary Research Center for Catalytic Chemistry, National Institute of
Advanced Industrial Science and Technology (AIST), Ibaraki 305-8565, Japan
d. Fuzhou University, College of Chemical Engineering, Fuzhou 350116, China
e. Department of Chemical Engineering, Virginia Tech, Blacksburg, VA 24061, United
States
† Electronic Supplementary Information (ESI) available: See
DOI: 10.1039/x0xx00000x
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intermediate temperatures so far. Herein, we report that calculated by Eqns. (S-1) and (S-2) in the ESI†. The conversion
View Article Online
DOI: 10.1039/D0CC05111J
ethane was converted directly to acetaldehyde and ethanol increased from 0.20% to 3.1% with increasing applied voltage.
with high production rates using oxygen species generated by The selectivity to C2 species such as C2H4, CH3CHO and C2H5OH
water electrolysis at 220oC.
increased, whereas the selectivity to undesirable CO2
decreased. The production rates of the products are shown in
Fig. S6. The production of the C2 species was drastically
promoted in the higher voltage region. Current densities at each
applied voltage were almost constant during the test as shown
in Fig. S7.
Results and discussion
Fig. 2 (a) The effect of applied voltage on the ethane conversion and the
product selectivity. (b) The effect of the amount of supplied oxygen atoms
generated by water splitting or fed as gaseous O2 on CH3CHO production rate.
Ethane flow rate: 3 mL min-1, temperature: 220oC, Pt weight: 1 mg cm-2.
Because the electrochemical cell consists of carbon-
containing materials such as the Pt/C electrodes and the current
collectors, there was a possibility for these materials to be
sources of the products in the ethane oxidation test.
Consequently, blank experiments were conducted to confirm
that the products were synthesized from the feed ethane on the
Pt/C anode. Table 1 summarizes the production rates of the C2
species at different experimental conditions. For both Pt/C
anode and carbon paper (CP) anode cases, no C2 species were
produced when a current density of 50 mA cm-2 was applied
without ethane gas (Entry 1 and 4). Little amounts of C2H4 were
detected at open circuit conditions in ethane flow (Entry 2 and
5). When current density of 50 mA cm-2 was applied with ethane
gas, the Pt/C anode and the CP anode showed different
performance. In the case of the Pt/C anode (Entry 3), significant
amounts of C2 species were produced. The amount of produced
C2 species, especially C2H4 and CH3CHO, were much larger in the
Pt/C anode case (Entry 3) than in the CP anode case (Entry 6).
Fig. 1 (a) Conceptual diagram of ethane partial oxidation in the electrochemical
cell. (b) A schematic illustration of ethane oxidation in the anode. SEM images
of (c) the phosphate-based electrolysis cell and (d) the interface between the
electrolyte and the Pt/C anode before the reaction.
The construction of the electrochemical cell is explained in the
Electronic Supplementary Information (ESI†). Briefly, a mixture
of CsH2PO4 and SiP2O7 powder was sandwiched between two
commercial Pt/C sheets (10 mm in diameter, Pt loading 1 mg
cm-2, where the Pt/C catalyst was loaded on carbon paper) in a
uniaxial pelletizer. XRD patterns for CsH2PO4 and SiP2O7 (Fig. S1,
ESI†) show that the two powders were successfully synthesized.
Figs. 1c and 1d show SEM images of the prepared cell. The
thickness of the cell was ~ 0.85 mm. The diameter of the
secondary particles of Pt/C was ~ 5 µm. Rod-shaped electrolyte
particles (~ 100 nm) and primary Pt/C particles (~ 50 nm) were
also observed (Fig. S2, ESI†). A SEM image after the ethane
oxidation (Fig. S3, ESI†) showed that the cross
Table 1 Comparison of the production rates with different conditions.
section of the electrolyte composite became dense
and the rod structure disappeared. This suggests
that the electrolyte material was in a highly viscous
molten state at around 200oC to achieve high H+
conductivity.17 The electrochemical H+ pumping
ability of the cell was examined with an I-V test and
confirmed to be sufficient (Fig. S4, ESI†). The
Faradaic efficiency for water splitting was 93%
(Table S1, ESI†). The electrochemical partial
oxidation of ethane was conducted at 220oC at
Gas condition
(steam, N2)
Current density
[mA cm-2]
Production rate
[10-9 mol s-1 cm-2]
Entry
Anode
C2H4 C2H5OH CH3CHO
1
Without C2H6
With C2H6
50
0
0
0
0
0
0
Pt/C
2
0.017
0.11
3
With C2H6
50
< 0.1
0.71
ambient pressure using a double-chamber reactor
(Fig. S5, ESI†).
Fig. 2a shows the effect of applied voltage on the
ethane conversion and the selectivity to the
products (CO2, CO, C2H4, CH3CHO, CH4 and C2H5OH)
4
Without C2H6
With C2H6
50
0
0
0
0
0
0
0
0
Carbon
paper
5
6
0.012
0.013
With C2H6
50
2 | J. Name., 2012, 00, 1-3
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The main product over the CP anode was CO2 and O2 (Entry 4
and 6, Table S5, ESI†). These results indicate that C2H4, C2H5OH
and CH3CHO are electrochemically produced in the electrolysis
cell from the feed ethane. Moreover, Pt metal has an important
role for the ethane oxidation to the oxygenates at intermediate
temperatures. The electrochemically-generated O species
would react with ethane on the Pt metal at the triple-phase
boundary (gas-electrode-electrolyte) as illustrated in Fig. 1b,
resulting in the production of the C2 species.
View Article Online
DOI: 10.1039/D0CC05111J
Fig. 2b shows the comparison of two types of oxidizing
species: gas phase O2 molecules and surface O species derived
from water splitting. CH3CHO production rates are plotted
against the number of supplied oxygen atoms. The production
rates of the other products are shown in the SI (Fig. S8, ESI†). In
addition, Faradaic efficiency for the products are summalized in
Tables S2 and S3. Applied voltages were almost constant during
the reaction (Fig. S9, ESI†). The production rate of CH3CHO
increased with the increase of the oxygen atoms supplied by
water electrolysis. On the other hand, gas phase O2 was not
consumed to synthesize CH3CHO, but was involved in the
formation of CO2 (Fig. S10, ESI†). This result indicates that
CH3CHO was produced by the electrochemically-generated O
species, not by gaseous O2. The production rates of the products
obtained under different steam partial pressures and a constant
voltage (Fig. S11, Table S4, ESI†) supports the discussion about
the oxidizing species. The production rates of the oxygenates
increased monotonically with the increase of current density,
indicating that the O species originating from the water splitting
worked as an effective oxidant. This is consistent with the
results in Figs. S6, S8, and S11 that once the electrochemically-
generated O species was used to form molecular oxygen, the
resultant molecular oxygen remained unreacted, and was
consequently detected as one of the products. The dependency
of the acetaldehyde production rate on the partial pressure of
ethane and the concentration of the O species was analyzed by
assuming the reaction rate equation below (Eqn. (1)).
Fig. 3 (a) Ethanol oxidation test. The production rates of C2H4, C2H6 and
CH3CHO as a function of current density. Ethanol flow rate: 0.8 mL min-1. (b)
Acetaldehyde oxidation test. The production rates of C2H4, C2H6 and C2H5OH
as a function of current density. Acetaldehyde flow rate: 10 mL min-1,
temperature: 220oC, Pt weight: 1 mg cm-2. (c) Schematic representations of
electrochemical ethane oxidation to C2H4, C2H5OH and CH3CHO.
α < 1 (Details are explained in the ESI†). The obtained kinetic
order for ethane (0.31) is in the range, being consistent with the
assumption that absorbed ethane species reacts with the O
species.
To investigate the reaction pathway in the anode,
electrochemical reactions starting from ethanol or
acetaldehyde were conducted. Fig. 3a shows the production
rates of the C2 species at different current densities in the
ethanol oxidation test. The production rates of the other
products are shown in the ESI† (Fig. S13). The production rates
of C2H4 and C2H6 increased as the current increased. CH3CHO
was produced even in the open circuit condition and its
production rate increased by applying current. It is suggested
that a certain amount of CH3CHO is produced from C2H5OH on
Pt/C catalytically at an open circuit according to Eqn. (2).15
푟CH CHO = 푘[C2H6]훼[O ∗ ]훽
(1)
3
푟
CH3CHO
, k, [C2H6], [O*], α and β indicate the production rate
C2H5OH → CH3CHO + H2
(2)
of acetaldehyde, rate constant, concentration of ethane,
concentration of the electrochemically-generated O species,
kinetic order for ethane and kinetic order for the
electrochemically-generated O species, respectively. Reaction
test results with different ethane partial pressures are shown in
Fig. S12. The kinetic orders, α and β were determined based on
the experimental data (Figs. S11 and S12, ESI†). Details of the
assumptions and the calculations are shown in the ESI†. The
resultant values of α and β were 0.31 and 1.6, respectively.
Based on the value of β higher than one (1.6), it is assumed that
more than one O atom is involved in the reaction: one O atom
is used to abstract hydrogen atom from ethane and another O
atom is inserted into the hydrocarbon fragment. This
assumption corresponds with the ethane oxidation steps with
the aid of monoatomic oxygen species.15 The experiment
without the Pt/C electrode shown in Table 1 (Entry 6)
demonstrates that the reaction between C2H6 and the O species
takes place on the Pt/C electrode not in the gas phase. By
considering the ethane adsorption on the catalyst surface, the
kinetic order for ethane, α, is estimated to be in a range of 0 <
Because H2O was not introduced to the anode in the ethanol
oxidation test, H+ producition from C2H5OH is requisite for
anodic current generation. Thus, the reactions shown in Eqns.
(3) and (4) might occur, and as a consequence C2H4 and CH3CHO
are produced. This indicates that ethanol generated from
ethane is easy to be oxidized under the anodic polarized
condition.
C2H5OH → C2H4 + 2H+ + O* + 2e-
(3)
(4)
C2H5OH → CH3CHO + 2H+ + 2e-
In the acetaldehyde oxidation test (Fig. 3b), miniscule amounts
of C2 species were generated from acetaldehyde, indicating
that acetaldehyde was not consumed to synthesize the C2
species under the anodic polarized conditions. Based on the
results shown in Figs. 2a, 3a and 3b, two reaction pathways are
considered for the ethane oxidation to the C2 species. One is
competitive oxidation of C2H6 to C2H4 and CH3CHO, where
C2H5OH is an intermediate product (Fig. 3c (I)). The other is non-
oxidative electrochemical deprotonation of C2H6 to C2H4 (Fig. 3c
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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Table 2 Comparison of the production rates on a catalyst weight basis.
(II)). These two reaction pathways
agree with those reported previously
(the competitive oxidation11 and the
electrochemical deprotonation23). The
more the cell was anodically polarized,
the more the production of C2H4 was
promoted (Figs. 2a and 3a). This
indicates that the competitive oxidation
to C2H4 and the deprotonation of C2H6
are dominant under highly-polarized
conditions. Thus, in order to suppress
the by-production of C2H4 and to
synthesize the oxygenates selectively at
the intermediate temperature, it is
View Article Online
DOI: 10.1039/D0CC05111J
Production rate
[μmol g-cat-1 s-1]
Temperature
[oC]
Applied current
[mA]
Catalyst
Oxidant
Ref.
C2H4
0
C2H5OH CH3CHO
80 [a]
Nafion-H
Pt
O2
40
0.26
0.54
14
79 [c]
160 [d]
2.8
5.2
0
1.2
16
This
220 [a]
H2O
0.42
work
475 [a]
50 [b]
Au
O2
H2O2
N2O
O2
5.3
0
-
0
0.16
-
15
10
11
12
13
H-LTA-Pt [e]
FePO4
-
-
-
-
0.0097
400 [b]
550 [b]
650 [b]
1.4
0.26
0.50
0.23
0.47
0.84
0.039
preferable
to
operate
the
B/BPO4
-
-
electrochemical cell under moderate
polarization.
V2O5/O-dia [f]
CO2
Table 2 compares the production
rates of the C2 species (C2H4, C2H5OH
[a] Electrochemically. [b] Thermal catalytically. [c] 30% steam condition applying 100 mA cm-2. [d] 40%
steam condition applying 10 V. [e] Pt containing LTA zeolite. [f] V2O5 supported on oxidized diamond.
and CH3CHO) using various catalysts in
thermal catalytic systems and in
5
6
7
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of other products are summarized as well in Table S6.
8
9
Conclusions
The partial oxidation of ethane was carried out using a
phosphate-based electrolyte and Pt/C electrodes at 220oC. The
ethane conversion and the selectivity to acetaldehyde and
ethanol increased under anodically polarized conditions. It was
confirmed that the C2 species were produced electrochemically
from ethane on the Pt/C electrode. Moreover, it was found that
the O species generated by water electrolysis functioned as an
effective oxidant for the synthesis of the oxygenates. This
report offers for the first time the possibility of ethane
conversion to oxygenates at intermediate temperatures using
the phosphate-based electrolyte and the electrochemically-
generated oxidants.
J. G. Vitillo, A. Bhan, C. J. Cramer, C. C. Lu, L. Gagliardi, ACS
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Conflicts of interest
There are no conflicts to declare.
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2008, 179, 497-503.
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4 | J. Name., 2012, 00, 1-3
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Acetaldehyde & ethanol are produced at high rates by
ethane partial oxidation at 220ºC & atmospheric pressure
using electrochemical cells.