Aqueous-Phase Selective Oxidation of Methane with Oxygen over Iron Salts and Pd/C in the Presence of Hydrogen

Article abstract of DOI:10.1002/cctc.201900919

Direct conversion of methane into value-added chemicals is a challenging but worthwhile subject. In this communication, the direct conversion of methane into methane oxygenates was achieved in an aqueous solution at room temperature using iron salts and Pd/C as catalysts and hydrogen peroxide as an oxidant. The hydrogen peroxide can be directly added or generated in situ from hydrogen and oxygen. The Pd/C catalyst greatly enhanced the reaction rate together with the iron salts. The effect of some parameters such as reaction temperature, reaction time, pH, acid type, and catalyst amount on the yields of the methane oxygenates was also investigated. When hydrogen peroxide was directly added, the turnover frequency (TOF), defined as the moles of methane oxygenates per moles of Fe per unit time, at 293 K was 29 h^?1 at pH=2.3. The TOF at 293 K was 42 h^?1 at pH=1.3 with in situ generated hydrogen peroxide from hydrogen and oxygen.

Full text of DOI:10.1002/cctc.201900919

Accepted Article
Title: Aqueous-phase selective oxidation of methane with oxygen over
iron salts and Pd/C in the presence of hydrogen
Authors: Jongkyu Kang and Eun Duck Park
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ChemCatChem
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COMMUNICATION
Aqueous-phase selective oxidation of methane with oxygen over
iron salts and Pd/C in the presence of hydrogen**
Jongkyu Kang and Eun Duck Park*
[
16-26]
,[12-15]
,[22]
,[24-
have
Abstract: Direct conversion of methane into value-added chemicals
is a challenging but worthwhile subject. In this communication, the
direct conversion of methane into methane oxygenates was
achieved in an aqueous solution at room temperature using iron salts
and Pd/C as catalysts and hydrogen peroxide as an oxidant. The
hydrogen peroxide can be directly added or generated in situ from
hydrogen and oxygen. The Pd/C catalyst greatly enhanced the
reaction rate together with the iron salts. The effect of some
parameters such as reaction temperature, reaction time, pH, acid
type, and catalyst amount on the yields of the methane oxygenates
was also investigated. When hydrogen peroxide was directly added,
the turnover frequency (TOF), defined as the moles of methane
trifluoroacetic acid (TFA)
using SO
3
K
2
S
2
O
8
H
2
O
2
2
5]
[20]
and O
2
as an oxidant. CH OSO H and CF COOCH
mostly been used as methanol precursors, which can be
transformed into methanol via hydrolysis, in either H SO or TFA.
3
3
3
3
2
4
Although high yields to obtain the methanol precursors can be
achieved in protic solvent systems, the requirements of strong
acids, expensive oxidants, and excessive amounts of water in
the hydrolysis step are still critical problems. To replace the
expensive oxidants with dioxygen, some homogeneous catalysts
were evaluated but failed to achieve high turnover frequencies
(TOFs).[20] The in situ generation of H
O
2
was also tried in an
2
effect to utilize dioxygen in methane functionalization.[
However, all of these reactions were performed in strong acid
solvent systems causing the dissolution of Pd metal which is
23,25]
-
1
oxygenates per moles of Fe per unit time, at 293 K was 29 h at pH
-1
=
2.3. The TOF at 293 K was 42 h at pH = 1.3 with in situ generated
hydrogen peroxide from hydrogen and oxygen.
2 2
responsible for the in situ generation of H O . Recently, selective
formation of methanol and formic acid was reported over Au-Pd
colloids,[ AuPd/TiO
5
26]
,
[27]
ZSM-5,
[28]
Fe/ZSM-5,
[29]
Cu-Fe/ZSM-
2
Methane is considered as a clean energy source because it
,[30] and Pd/ZSM-5
[31]
in an aqueous solution with H O .
2 2
2
emits the smallest amount of CO owing to the fact that it has the
However, the selective oxidation of methane with dioxygen as an
oxidant with high TOFs has not been reported in an aqueous
solution. In this study, we report that formic acid can be produced
highest H/C ratio among hydrocarbons. A recent upsurge in
shale gas production has spurred research and development in
the utilization of methane as a chemical feedstock.[1] Currently
methane is converted into various chemicals indirectly via
from methane in the presence of an iron salt and Pd/C with H
2 2
O
or a mixture of H and O . The TOF is dependent on the pH of
2
2
syngas, a mixture of CO and H
typically through an energy intensive steam reforming reaction
CH + H O ↔ CO + 3H
, ΔHo298K = 206 kJ/mol) at high
2
, which can be synthesized
the aqueous solution but the reaction conditions are much milder
than the previous protic solvent systems.
(
4
2
2
The combination of FeSO
particle size of 3.5 nm (Fig. S1), was effective for the selective
methane oxidation with H (Table 1, entry 1) to produce
HCOOH and CH OOH. Only trace amounts of methane
oxygenates were detected using FeSO with H (Table S1,
entry 1). However, no methane oxygenates were obtained using
Pd/C with H (Table S1, entry 2) indicating that there is a
synergistic effect between FeSO and Pd/C. The decomposition
of H was accelerated in the presence of Pd/C (Table S1). The
4
and Pd/C, with an average Pd
temperatures. To circumvent this energy-consuming process,
the direct conversion of methane has been sought in different
ways but these effects have so far failed to be cost-competitive
with an indirect pathway.[2] In particular, the direct oxidation of
methane into methane oxygenates such as methanol,
formaldehyde, and formic acid under mild conditions is still
challenging.[3,4]
2
O
2
3
4
2 2
O
2
O
2
4
The gas-phase partial oxidation of methane has been reported
2
O
2
over Fe- or Cu-zeolites[5,6] using N
O,
[7]
O ,
2
[8]
and H
O
[9,10]
as
2
2
decomposition of the carbon support in Pd/C during reaction can
be excluded because there was no oxygenated carbon species
in the liquid and gas phase without methane (Table S1, entry 3).
In order to find out the effect of iron salts, various iron salts were
evaluated in the presence of Pd/C. All of the iron salts were
oxidants. However, most reactions can only proceed at relatively
[
11]
high temperatures (> 200 ℃) resulting in low product yields.
On the other hand, much higher yields of methane oxygenates
have been reported in a liquid-phase system.[3,4] Various
homogeneous metal catalysts such as Hg,[12] Pt,[13-15] Pd,[16,17]
active in this reaction in the presence of H
Instead of H , a gas mixture of H
generate in situ to oxidize methane into methane
oxygenates. The gas composition was fixed to produce
theoretically the same amount of H added directly in Table 1,
which can be obtained only when hydrogen was converted into
with a 100% yield. Interestingly, much higher yields of
methane oxygenates were achieved with in situ generated H
Table 2) than with directly added H (Table 1). To investigate
2
O
2
(Table 1).
Eu,[18] Rh,[19] Co,[20] Os,[21] Cu,[22,23] V,[23,24] and Fe[25,26] have been
2
O
2
2
and O was used to
2
SO [
12-15]
and
reported to be active in a protic solvent such as H
2
4
2 2
H O
2
O
2
[
[
*]
J. K. Kang, Prof. Dr. E. D. Park
Department of Chemical Engineering and Department of Energy
Systems Research, Ajou University
2 2
H O
2
O
2
206 World cup-ro, Yeongtong-Gu, Suwon 16499, Republic of Korea.
(
2 2
O
E-mail: edpark@ajou.ac.kr
**]
Supporting information, including catalyst testing, analytical
protocols, materials and methods, catalyst characterization
techniques, and additinal results can be found under:
https://doi.org/***
the reason for the increased yield in ‘in situ conditions’, controlled
experiments were conducted by changing the gas composition
in the case of the direct H
, 5, 6 and 7). The positive effect of H
yield was observed only in the copresence of FeSO
Table S1, entry 6). It should be noted that the product yields of
2
O
2
injection system (Table S1, entries
on the methane oxygenate
and Pd/C
4
2
4
(
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ChemCatChem
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COMMUNICATION
2 2
Table 1: Catalytic performance for the selective oxidation of methane over iron salts and Pd/C with H O
.[a]
Product [μmol]
HCOOH[b]
CH
Methane
oxygenates
selectivity [%]
85 ± 6
[
Total
2 2
H O
conversion [%]
Entry
Iron salt
OH[b]
OOH[b]
CO
[c]
[e]
CH
3
3
2
[d]
products]/[Fe]
1
2
3
4
5
6
FeSO
Fe (SO
FeCl
FeCl
Fe(NO
Fe(CH COO)
4
5 ± 0
7 ± 1
6 ± 0
6 ± 1
5 ± 1
5 ± 2
36 ± 4
67 ± 2
38 ± 3
23 ± 3
70 ± 3
69 ± 4
19 ± 3
17 ± 2
22 ± 5
22 ± 0
18 ± 5
22 ± 2
11 ± 4
12 ± 5
7 ± 1
3 ± 0
10 ± 0
12 ± 2
6.3 ± 0.2
9.1 ± 0.8
6.5 ± 0.6
4.7 ± 0.3
9.1 ± 0.7
9.5 ± 0.2
40 ± 7
51 ± 2
32 ± 4
25 ± 1
39 ± 1
46 ± 6
2
)
4 3
89 ± 4
90 ± 2
95 ± 1
90 ± 1
2
3
)
3 3
3
2
90 ± 2
[
=
a] Standard reaction conditions: Vliquid = 30 mL, Vgas = 95 mL, [H
20 ℃, reaction time = 30 min. [b] Analyzed by H-NMR, TMSP was used as a reference. [c] Analyzed by GC-FID. [d] (moles of methane oxygenates except for
2
O
2
2 4
]= 0.28 M, [H SO ] = 15 mM, [Fe] = 0.37 mM, 1% Pd/C 50 mg, PCH4 = 15 bar, PN2 = 13 bar, T
1
2 2 2
CO )/(moles of total methane oxygenates including CO )× 100. [e] (moles of total methane oxygenates including CO )/(moles of Fe added). The numbers in the
table are mean values after two iteration, and the numbers in parentheses indicate the error range.
methane oxygenates in the direct H
S1, entry 6) are comparable to those in the in situ H
system (Table 2, entry 1). There was a slight increase in the
product yield in the presence of oxygen in the direct H injection
system (Table S1, entry 7). In the case of the in situ H
2
O
2
injection system (Table
S1) were also examined for this reaction with FeSO
formation of methane oxygenates was observed for the catalyst
system composed of FeSO and either Pt/C or Rh/C irrespective
of the presence of H or a gas mixture of H and O (Tables S5
and S6). In the case of direct H injection system, Pt/C and
4
. The
2
O
2
generation
4
2
O
2
2
O
2
2
2
2
O
2
2 2
O
generation system, the possibility of oxidation of the carbon
support to oxygenates can be excluded because no product was
obtained in the absence of methane, (Table S2, entry 1).
Rh/C showed comparable methane oxygenate yields. However,
Pt/C provided a much higher yield of methane oxygenates than
Rh/C in the in situ H
related to the different production rates of H
over these two metal catalysts. It is worth mentioning that Pt/C
gave a higher methane oxygenate yields in the in situ H
generation system than in the direct H injection system. On
the other hand, Rh/C provided a higher methane oxygenate yields
in the direct H injection system than in the in situ H
generation system. In any case, the yields of methane oxygenates
over Pt/C + FeSO or Rh/C + FeSO were much lower than those
over Pd/C + FeSO . The other supported Pd catalysts such as
Pd/SiO and Pd/ZSM-5 were also active for selective methane
oxidation with FeSO in the presence of H and O (Table S7).
2
O
2
generation system. This might be closely
Additionally, if any of the two reactant gases (H
substituted by N , no product was obtained (Table S2). In order to
find out the effect of iron salts, various iron salts were also
evaluated in the presence of Pd/C with H and O . All of the iron
salts were active in this reaction in the presence of a gas mixture
of H and O (Table 2). In any case, the in situ generated H
was better at achieving a higher yield of methane oxygenates
Table 2) than directly added H (Table 1). This is a remarkable
result because all of the H and O
2
and air) was
2
O
2
from H and O
2
2
2
2
O
2
2
2
2 2
O
2
2
2
O
2
2
O
2
2 2
O
(
2
O
2
4
4
2
2
cannot be transformed into
4
H
2
O
2
with a 100% yield under these reaction conditions.[ This
32]
2
implies that the presence of hydrogen exerts a positive effect on
the catalytic activity in both systems as revealed in the direct H
injection system (Table S1, entry 6). The residual concentrations
of H in the in situ system (Table 2) were low because the
hydrogen peroxide formed from H and is consumed
simultaneously to convert methane. The lower methane
oxygenates selectivity was obtained in in situ H generation
system (Table 2) than in the direct H injection system (Table
).
4
2
2
2
O
2
However, the yields of methane oxygenates were much lower
than those of Pd/C. The yield of methane oxygenates appears to
increase with decreasing Pd particle size (Fig. S1) which is
2
O
2
2
O
2
reported to be closely related to the H
The effect of the metal content of the iron salts and Pd/C on the
catalytic activity was evaluated in the direct H injection or in
situ H generation system. The yield of methane oxygenates
increased with the amount of FeSO and Pd/C in both cases
(Tables S8, S9, S10, and S11). In the direct H injection system,
the H conversion generally increased with the amount of
FeSO and Pd/C (Tables S8 and S9). The turnover number (TON)
2 2
O
formation rate.[33, 34]
2
O
2
2 2
O
2
O
2
2 2
O
1
4
Other transition metal salts were examined under the same
reaction conditions. Among the tested catalysts, no transition
metal salt showed a comparable catalytic activity with the iron
2 2
O
2
O
2
4
salts in the direct H
2
O
2
injection or in situ H
2
O
2
generation system
based on the moles of Fe increased with the amount of Pd/C and
(
Tables S3 and S4) indicating that this reaction is uniquely
reached a maximum value (Table S9). In the case of the in situ
performed over iron salts in the presence of Pd/C. Other noble
metal catalysts such as Pt/C with an average Pt particle size of
2 2
H O generation system, the selectivity for methane oxygenates
decreased with an increasing amount of Pd/C or FeSO
S10 and S11).
4
(Tables
2
.1 nm and Rh/C with an average Rh particle size of 2.8 nm (Fig.
2 2
Table 2: Catalytic performance for the selective oxidation of methane over iron salts and Pd/C with H and O
.[a]
Product [μmol]
Methane oxygenates
selectivity [%][d]
66 ± 1
Residual concentration
of H [mM]
[Total
products]/[Fe]
32.1 ± 0.6
32.7 ± 1.4
31.5 ± 0.4
27.5 ± 3.5
33.4 ± 2.2
33.0 ± 1.4
Entry
Iron salt
OH[b]
HCOOH[b]
CH
3
OOH[b]
CO
[c]
[e]
CH
3
2
2 2
O
1
2
3
4
5
6
FeSO
Fe (SO
FeCl
FeCl
Fe(NO
Fe(CH COO)
4
11 ± 1
10 ± 1
9 ± 0
10 ± 0
8 ± 0
218 ± 0
216 ± 2
7 ± 1
8 ± 1
124 ± 8
132 ± 18
137 ± 3
83 ± 10
139 ± 19
131 ± 3
12
16
18
16
18
13
2
)
4 3
64 ± 4
61 ± 0
73 ± 0
63 ± 3
2
198 ± 2
9 ± 0
3
205 ± 29
218 ± 5
10 ± 1
9 ± 1
)
3 3
3
2
10 ± 0
218 ± 10
10 ± 2
64 ± 1
[
2
a] Standard reaction conditions: Vliquid = 30 mL, Vgas = 95 mL, [H
0 ℃, reaction time = 30 min. [b] Analyzed by H-NMR, TMSP was used as a reference. [c] Analyzed by GC-FID. [d] (moles of methane oxygenates except for
2
SO
4
] = 15 mM, [Fe] = 0.37 mM, 1% Pd/C 50 mg, PCH4 = 15 bar, PH2 = 3 bar, Pair = 10 bar, T =
1
2 2 2
CO )/(moles of total methane oxygenates including CO )× 100. [e] (moles of total methane oxygenates including CO )/(moles of Fe added). The numbers in the
table are mean values after two iteration, and the numbers in parentheses indicate the error range.
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ChemCatChem
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COMMUNICATION
situ H
system. This implies that the catalytic cycle can be facilitated in
the in situ H generation system compared to the direct H
injection system. Under the same reaction conditions, CH
was converted into HCOOH and CO (Table S17). HCOOH was
2 2 2 2
O generation system than in the direct H O injection
4
3
2
1
00
00
00
00
0
2
O
2
2 2
O
Black: CO
2
Colored: methane
oxygenates
3
OH
2
easily transformed into CO
consecutive reactions from methane to CO
CH OH, and HCOOH are prevalent under these reaction
2
(Table S17). This implies that the
2
through CH OOH,
3
3
conditions. The product yields were also dependent on the
reaction temperature. The higher yields of methane oxygenates
were achieved with an increasing reaction temperature from 10 to
30 ℃ in both cases (Tables S18 and S19). However, the
selectivity for methane oxygenates decreased with increasing
temperature because the further oxidation reactions of the
methane oxygenates are accelerated more with increasing
reaction temperature.
0.7
1.4
2.3
pH
3.4
4.3
Figure 1. Effects of pH on the selective oxidation of methane over FeSO
4
and
] (blue).[ . *Standard reaction conditions:
2 2
O
a]
When ethane was used as a reactant, ethane oxygenates due
Pd/C with [H
2
+ O
2
] (green) and [H
SO
V
liquid = 30 mL, Vgas = 95 mL, [H
2
4
] = 0.015 mM – 150 mM, [Fe] = 0.37 mM,
% Pd/C 50 mg, T = 20 ℃, reaction time = 30 min. Reaction with [H + O ]:
]=0.28 M,
to C-H activation (C
2 5 3
H OH and CH COOH) as well as methane
1
2
2
oxygenates owing to C-C scission and C-H activation (CH OH,
3
P
P
CH4 = 15 bar, PH2 = 3 bar, Pair = 10 bar. Reaction with H
CH4 = 15 bar, PN2 = 13 bar
2 2 2 2
O : [H O
3
HCOOH, CH OOH) were obtained (Table S20). This implies that
radical species as nonselective intermediates must be involved in
this reaction system. In Fenton chemistry, the hydroxyl radical is
The effect of pH on the catalytic performance was also examined
Tables S12 and S13). In the case of direct H injection system
the total product yield to methane oxygenates increased with
decreasing pH from 4.3 to 2.3 and showed a maximum value at
pH of 2.3. This then decreased with further decreasing pH (Fig. 1
blue, Table S12). A similar trend was also observed in the case
reported to be formed via the reaction between Fe2 and H
+
2
2
O ,
(
2 2
O
which is as follows:[40]
Fe2+ + H → Fe3+ + HO· + OH- (1)
O
2 2
This formed hydroxyl radical (HO·) is active enough to break down
the C-H bond in methane to form the methyl radical as follows:
CH
4
+ HO· → CH
3
· + H
2
O
(2)
of the in situ H
However, the maximum yield was obtained at a lower pH of 1.4
than in the case of the direct H injection system. This is clearly
related to the stability of H which is strongly affected by pH. In
2 2
O generation system (Fig. 1 green, Table S13).
A further radical-initiated reaction can be carried out to form
CH
3
OOH, CH
3
OH, HCOOH, and CO
2
.[26] However, the reduction
2 2
O
of Fe3 into Fe2+ should occur easily under reaction conditions to
+
2
O
2
increase the turnover number. Very low methane oxygenate
Fenton chemistry, it is generally accepted that iron hydroxide can
yields obtained in the presence of FeSO
this reduction of Fe3+ into Fe2+ does not occur spontaneously
under the given reaction conditions. The aqueous FeSO solution
4 2 2
and H O indicate that
be formed at a pH above 4, which is not favorable for the
generation of hydroxyl radical species.[35-37] A lower pH is
favorable for stabilizing H O but the formation of H O
2 2 3 2
+, which
occurs at a pH below 2, is not desirable for the formation of
reactive hydroxyl radical species.[35-37] A close correlation can be
4
in 15 mM sulfuric acid has a low absorption band in the
wavelength range of 300 – 400 nm in UV-Vis spectra (Fig. S2).
On the other hand, a strong absorption band can be observed for
found between the total product yield and H
direct H injection system. It should be noted that the selectivity
for methane oxygenates increased with decreasing pH in both
2 2
O conversion in the
the aqueous Fe
There was no change in the absorption band due to Fe even
after injection of H into the Fe (SO solution (Fig. S2A). On
the other hand, once H is added to the aqueous FeSO
solution, the absorption band appears indicating the oxidation of
Fe2+ into Fe3+ (Fig. S2B). In the presence of H
and Pd/C,
2 4 3
(SO ) solution in 15 mM sulfuric acid (Fig. S2).
2
O
2
3
+
2
O
2
2
4 3
)
systems. The preferential formation of CO
2
with increasing pH
above 2 is in line with the previous report by Duesterberg et al.,[
38]
2
O
2
4
in which the reaction rate between formic acid and hydroxyl
radical to form CO increased with increasing pH from 2.5 to 4.0.
2
It is worth mentioning that the fraction of leached Pd from Pd/C
was determined to be less than 2% of the total Pd in the in situ
2
dihydrogen can be easily dissociated into atomic hydrogen over
Pd/C and the reduction of Fe3+ into Fe2+ can be facilitated (Fig.
S3), which results in the production of reactive hydroxyl radicals
to activate methane (eqn. (2)).
2 2
H O generation system even at the lowest pH of 0.7, which
indicates that the Pd/C system is relatively stable under these
reaction conditions. The effect of acid to control the pH was also
examined. The pH was fixed at 1.3 and various protic acids were
H
2
+ 2* ↔ 2H*
(3)
H* + Fe3+ → * + Fe + H+ (4)
2+
The dissociation of molecular hydrogen into atomic hydrogen can
occur over noble metals such as Pt, Pd, and Rh. In the presence
compared for this reaction. Both H
in high yields of methane oxygenates among the tested acids
Table S14). On the other hand, HCl was inferior to the above
2 3
SO4, HNO and TFA resulted
of hydrogen and oxygen, H
oxygen and dissociated hydrogen atoms as follows.
H* + O ↔ H + 2* (5)
2 2
O can be produced from molecular
(
mentioned acids indicating that the anion can affect the catalytic
activity for this reaction (Table S14). HCl was reported to
scavenge hydroxyl radical in Fenton chemistry.[39] The product
distribution was monitored as a function of reaction time. The
2
2
2 2
O
This reaction is reported to be strongly dependent on the metal
and the activity of the metals generally decreased in the following
order: Pd > Pt >> Rh.[34, 41] This can explain why the methane
oxygenate yields depended on the noble metal catalysts in the in
yields of HCOOH and CO
other hand, the yields of CH
2
increased with reaction time. On the
OH and CH OOH appeared to be
3
3
2 2
situ H O generation system. Based on them, the reaction
maintained (Tables S15 and S16). It should be noted that the
turnover frequency based on a Fe atom (TOF) is higher in the in
scheme for the synthesis of methane oxygenates from methane,
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ChemCatChem
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COMMUNICATION
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[
[
[
[
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[
[
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2
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Jongkyu Kang and Eun Duck Park*
Methane-to-Oxygenates: Aqueous-
phase selective oxidation of methane
with molecular oxygen under mild
conditions was successfully
demonstrated using iron salts and
Pd/C in the presence of molecular
hydrogen
Page No. – Page No.
Aqueous-phase selective oxidation
of methane with oxygen over iron
salts and Pd/C in the presence of
hydrogen
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