Direct synthesis of oxygenates via partial oxidation of methane in the presence of O2 and H2 over a combination of Fe-ZSM-5 and Pd supported on an acid-functionalized porous polymer

Article abstract of DOI:10.1016/j.apcata.2020.117711

Aqueous-phase partial oxidation of methane over a combination of heterogeneous Fe-ZSM-5 and Pd catalysts was demonstrated in the presence of O₂ and H₂. Fe-ZSM-5 catalyst active for partial oxidation of methane with H₂O₂ was prepared by an ion-exchange method. For comparison, homogeneous Fe species (FeSO₄) were also used as a catalyst for partial oxidation of methane. Several supported Pd catalysts for the direct synthesis of hydrogen peroxide from H₂ and O₂ were prepared using activated carbon (AC), AC treated with sulfuric acid (s-AC), as-synthesized hyper-crosslinked porous polymer (HCPP), HCPP with carboxylic group (c?HCPP), and HCPP with carboxylic and sulfonic groups (c-s?HCPP). Among them, the Pd catalyst supported on c-s?HCPP was effective for generating H₂O₂ in situ, which enabled methane oxidation over Fe-ZSM-5 without any leaching.

Full text of DOI:10.1016/j.apcata.2020.117711

Journal Pre-proof
Direct synthesis of oxygenates via partial oxidation of methane in the
presence of O and H over a combination of Fe-ZSM-5 and Pd
2
2
supported on an acid-functionalized porous polymer
Jongkyu Kang (Validation) (Investigation) (Data curation) (Writing -
original draft), Pillaiyar Puthiaraj (Investigation) (Data curation),
Wha-seung Ahn (Writing - review and editing), Eun Duck Park
(
(
Conceptualization) (Writing - review and editing) (Supervision)
Project administration) (Funding acquisition)
PII:
S0926-860X(20)30304-5
DOI:
https://doi.org/10.1016/j.apcata.2020.117711
APCATA 117711
Reference:
To appear in:
Applied Catalysis A, General
Received Date:
Revised Date:
Accepted Date:
17 April 2020
9 June 2020
17 June 2020
Please cite this article as: Kang J, Puthiaraj P, Ahn W-seung, Park ED, Direct synthesis of
oxygenates via partial oxidation of methane in the presence of O and H over a combination
2
2
of Fe-ZSM-5 and Pd supported on an acid-functionalized porous polymer, Applied Catalysis
A, General (2020), doi: https://doi.org/10.1016/j.apcata.2020.117711

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Revised to Applied Catalysis A: General (Ms No. APCATA-D-20-00688)
Direct synthesis of oxygenates via partial oxidation of methane in the
presence of O
2
and H
2
over a combination of Fe-ZSM-5 and Pd supported
on an acid-functionalized porous polymer
a
b
b
a,c*
Jongkyu Kang , Pillaiyar Puthiaraj , Wha-seung Ahn , Eun Duck Park
aDepartment of Energy Systems Research, Ajou University, 206 Worldcup-ro Yeongtong-
gu, Suwon 16499, Republic of Korea
bDepartment of Chemistry and Chemical Engineering, Inha University, 100, Inha-ro,
Michuhol-gu, Incheon, 22212, Repub-lic of Korea
cDepartment of Chemical Engineering, Ajou University, 206, World cup-ro, Yeongtong-gu,
Suwon, 16499, Republic of Korea
Tel.: 82-31-219-2384;
Fax: 82-31-219-1612;
*
E-mail: edpark@ajou.ac.kr
*
To whom all correspondence should be addressed
Graphical abstratc

Highlights


Liquid-phase partial oxidation of methane with O was achieved in the presence of H
2
2
.
Pd supported on acid-functionalized porous polymer was used for in situ H
tion.
2
O genera
2


Fe-ZSM-5 was stable and active for methane oxidation with in situ H
2
O .
2
-1
The combination of these two catalysts showed the TOF of 3.8 h at 30 ℃.
ABSTRACT

Aqueous-phase partial oxidation of methane over a combination of heterogeneous Fe-ZSM-
and Pd catalysts was demonstrated in the presence of O and H . Fe-ZSM-5 catalyst active
5
2
2
for partial oxidation of methane with H
2
O
2
was prepared by an ion-exchange method. For
) were also used as a catalyst for partial oxidation
comparison, homogeneous Fe species (FeSO
4
of methane. Several supported Pd catalysts for the direct synthesis of hydrogen peroxide from
and O were prepared using activated carbon (AC), AC treated with sulfuric acid (s-AC),
H
2
2
as-synthesized hyper-crosslinked porous polymer (HCPP), HCPP with carboxylic group (c-
HCPP), and HCPP with carboxylic and sulfonic groups (c-s-HCPP). Among them, the Pd
catalyst supported on c-s-HCPP was effective for generating H
methane oxidation over Fe-ZSM-5 without any leaching.
2
2
O in situ, which enabled
Keywords: methane activation, heterogeneous catalysis, methane oxygenates, hydrogen
peroxide, liquid-phase oxidation
1
. Introduction
Direct conversion of methane into value-added products has been a challenge over the past
decades. Recent interest in new chemical feedstock other than coal and oil and growing shale
gas production has rekindled the need for direct conversion of methane [1]. Several
methodologies are reported for the direct conversion of methane, including aromatization,
coupling, pyrolysis, and partial oxidation of methane into oxygenates [2]. Among them, partial
oxidation of methane is particularly attractive in that it directly converts methane under mild
reaction conditions with less energy required than the syngas route.

The partial oxidation of methane in the gas phase has been frequently reported over Cu and
Fe catalysts supported on zeolites or SiO [3-6]. Generally, it requires N O, O , or H O as the
2
2
2
2
oxidant at a reaction temperature of 200 ℃or higher [7-12]. In comparison, the partial oxidation
of methane in liquid phase can be conducted at room temperature. Various noble metals [13-
1
8] such as Pd, Pt, and Rh and non-noble metals [19-29] such as Fe, Co, Cu, V, Os, Eu, and
Hg have been used as catalysts. Oxidants include SO , K , hydroperoxide (organic
peroxides and H ), and O with or without reducing agents [30-37]. However, oxidants
costing more than O are commonly utilized. Furthermore, protic acids such as sulfuric acid
3
2
S
2
O
8
2
O
2
2
2
and trifluoroacetic acid are generally used as solvents in order to stabilize methanol, which
consequently cause the instability of the heterogeneous catalysts used [38]. Aqueous-phase
methane oxidation was also reported over Fe-ZSM-5, Au-Pd/TiO
Pd/ZSM-5 using H [39-41]. Since H is an expensive oxidant, utilization of O
presence of H has also been investigated to oxidize methane in aqueous phase [42,43].
Recently, Xiao et al. reported that the concentration of H within the catalyst was enriched
2
, Au-Pd colloid and Au-
2
O
2
2
O
2
2
in the
2
2
O
2
through substitution of carbon chains on the outer surface of Au-Pd nanoparticles encapsulated
in ZSM-5, which led to an increase in methanol yield [44].
The direct synthesis of hydrogen peroxide from H
decomposition and hydrogenation occur simultaneously within the reaction [46,47]. Low pH
of reaction medium was reported to be beneficial for stabilization of H and inhibiting side
2
and O
2
has low product yields since
2
O
2
reactions [48]. However, the use of strong protic acids can cause corrosion of the reactor or
leaching of active metal into the reaction medium [49,50]. Thus, solid acids including insoluble
heteropoly acid [51], zeolites [52], acid- functionalized carbon material [53], metal oxide [54],
and acid-functionalized hyper-crosslinked porous polymer (HCPP) [45] have been applied to
overcome these drawbacks.

Based on the fact that Fe-ZSM-5 is quite effective for partial oxidation of methane in water
and that Pd catalysts supported on an acid-functionalized support are efficient for H
generation from H and O [45], herein we report our work combining these two catalytic
systems in order to oxidize methane over Fe-ZSM-5 with H generated in situ from H and
over a supported Pd catalyst (Scheme 1).
2
O
2
2
2
2
O
2
2
O
2
2
. Experimental
2
.1. Chemicals
All materials such as iron sulfate, palladium(II) chloride, palladium(II) acetate, hydrazine
hydrate, dichloromethane, 1,3,5-triphenylbenzene, 2,4,6-tris(bromomethyl)mesitylene,
anhydrous aluminum chloride, sodium hydroxide, potassium permanganate, chlorosulfonic
acid, sulfuric acid, Ferroin indicator and activated carbon (AC) were purchased from Sigma-
Aldrich. Methanol, ethanol, acetone, and hydrochloric acid were received from Merck
Millipore. Cerium(IV) sulfate was purchased from Kanto Chemical. Hydrogen peroxide
(
34.5%) was supplied by Samchun Pure Chemical. ZSM-5 (NH
4
-form, SiO
2
/Al
2
3
O = 30, SBET=
2
4
05 m /g) was received from Zeolyst. All of chemicals were purchased with a reagent grade
and used without further purification.
2
.2. Catalysts preparation
Fe-ZSM-5: 2.97 g of NH
nitric acid to adjust the pH to 3. After heating the solution to 80 ℃, a given amount of FeSO
O was added into the solution and stirred for 2 h. The sample was filtered, washed with
4
-ZSM-5 was added into 300 mL of deionized water containing
4
·
7
H
2

deionized water, and dried in an oven at 110 ℃ overnight. The dried sample was calcined in
static air at 550 ℃for 3 h before use.
Sulfuric acid-treated AC (s-AC): 5 g of AC was added to 50 mL of sulfuric acid in 500 mL
round bottom flask. The flask was heated to 160 ℃ and maintained for 1 h while stirring
vigorously. After 1 h, the liquid was cooled down to room temperature, filtered, washed with
hot water, and dried in an oven at 110 ℃.
1
wt.% Pd/AC: The 1 wt. % Pd/AC catalyst was prepared by a wet impregnation method
using palladium(II) chloride as a precursor. In this method, a given amount of PdCl was added
2
into 50 mL of deionized water with a few drops of HCl solution. Next, 4.95 g of AC was added
into the solution. After mixing the solution for 3 h at 60 ℃, water was evaporated using a rotary
evaporator. The catalyst was dried in an oven at 110 ℃ overnight and reduced with H
before use.
wt.% Pd/s-AC: The procedure was same with that of 1 wt.% Pd/AC only except that s-AC
was used instead of AC and it was reduced using NaBH solution.
2
at 300
℃
1
4
Hyper-crosslinked porous polymer (HCPP), carboxylic acid-functionalized hyper-
crosslinked porous polymer (c-HCPP), carboxylic and sulfonic acid-functionalized hyper-
crosslinked porous polymer (c-s-HCPP), Pd/HCPP, Pd/c-HCPP, and Pd/c-s-HCPP were
prepared by following the same procedure in the previous report [45]. The details are also
described in the supplementary information.
2
.3. Characterization
The textural properties of the supports and Pd catalysts such as Brunauer–Emmett–Teller
(
BET) surface area and pore volume were measured via N physisorption analysis at -196 °C
2

using a Micromeritics ASAP 2020 instrument. All samples were degassed under vacuum for 6
h at 200 ℃prior to analysis.
Elemental analysis (C, H, N, and S) were performed on Flash EA1112 (CE). The acid density
of HCPP, c-HCPP, c-s-HCPP, AC, and s-AC was determined by titration with NaOH solution
[
55]. 50 mg of the sample was suspended in 20 mL of 2 M aqueous NaCl solution. The resulting
suspension was stirred at room temperature for 24 h and titrated by addition of 0.1 M NaOH.
The presence of S species in s-AC was also confirmed by temperature-programmed
desorption with mass spectroscopy (TPD-MS) conducted on Autochem 2920 (Micromeritics)
with a mass spectrometric detector. 50 mg of AC or s-AC was loaded in a sample tube. After
purging the sample with helium for 30 min, temperature was increased to 900 ℃at a rate of 10
℃/min while monitoring the mass signal. m/z= 48, m/z=44, m/z=28, and m/z=18 was assigned
to SO , CO , CO, and H O, respectively.
2
2
2
The transmission electron microscopy (TEM) images were obtained using Tecnai G2 TEM
FEI) operating at 200 kV. The sample was finely ground and dispersed ultrasonically in
(
ethanol before deposition on a Cu grid covered by a perforated carbon film. The average
particle size of Pd in each supported Pd catalyst was determined based on the TEM images.
The metal content in the catalyst and the amount of metal leached in liquid was quantified
by inductively coupled plasma-optical emission spectrometer using OPTIMA 5300DV
(PerkinElmer).
2
.4. Catalytic activity evaluation
The selective oxidation of methane was carried out in an autoclave with a volume of 125
mL. This autoclave has an outer jacket connected to two separate circulators. One circulator is

maintained at the reaction temperature and the other is fixed at -10 ℃ to quench the reaction.
The glass liner was installed inside the autoclave to avoid the side reactions. Two different
systems were used in this study. The one is the direct H
2
O
2
injection system. 30 mL of an
was fed into the reactor
aqueous solution containing catalyst and certain concentration of H
2
O
2
and sealed. The reactor was charged with methane to 15 bar after a series of purges (3 times
with methane at 15 bar) to remove any contaminant gas in the reactor and then charged with
nitrogen to a fixed final pressure (typically 28 bar). The valve between the reactor and the
circulator at the reaction temperature was opened. Once the temperature of the aqueous solution
reached the target temperature (it took generally less than 10 min), the solution was mixed
vigorously at 1200 rpm with a magnetic stirrer. The reaction was performed for the desired
time (typically 30 min), after which the reactor was cooled by switching the valve to the
circulator at -10 ℃in order to minimize the loss of the products due to vaporization. The other
is the in situ H
2
2
O generation system. 30 mL of an aqueous solution containing catalysts, and
optionally acid to adjust pH was fed into the reactor and sealed. The reactor was charged with
methane to a fixed pressure (typically 15 bar) after a series of purges (3 times with methane at
1
5 bar) to remove any contaminant gas in the reactor and then charged with hydrogen to a
pressure of 18 bar and finally charged with air to a fixed final pressure (typically 28 bar).
2
.5. Analytic Protocols
The gas-phase products were analyzed online using a gas chromatograph (GC) (YL
Instrument 6100GC), which was equipped with a packed Carbosphere column for flame
ionization detection (FID). The GC was fitted with a methanizer unit and CO was quantified
2
according to the calibration curve obtained from commercial standards. After the reaction, the
1
solution was filtered and the liquid-phase products were analyzed by H-NMR spectroscopy.

1
NMR spectra were recorded at room temperature on a Jeol Resonance ECZ600R ( H 599.7
MHz), and quantified with a 0.1% trimethylsilylpropinoic acid (TMSP)/D O standard (Euriso-
top). Typically, 0.7 mL of sample and 0.1 mL of D O were placed in an NMR tube along with
the standard. A solvent suppression program was run in order to minimize the signal arising
from the solvent. The amount of H remaining after the reaction was quantified by the
titration method using Ce(SO and ferroin indicator.
2
2
2
O
2
)
4 2
3
. Results and discussion
3
.1. Characterization of the catalysts
The physical properties of the supports and Pd catalysts were measured and reported in Table
S1. The BET surface area of HCPP, c-HCPP, and c-s-HCPP was determined to be 1.84 x 103,
3
2
1
.45 x 10 , and 929 m /g, respectively. The pore volume of HCPP, c-HCPP, and c-s-HCPP was
3
determined to be 1.6, 1.3, and 0.7 cm /g, respectively. These results indicate that the porous
structure of porous polymer was partly degraded during the surface functionalization. On the
other hand, there was no significant change in the BET surface area and pore volume between
AC and s-AC. The BET surface area and pore volume of the supported Pd catalysts were
decreased a little compared with those of the support itself.
The acid density of c-HCPP, c-s-HCPP, and s-AC was determined to be 0.6, 1.5, and 0.2
mmol/g, respectively. Note that no acid site was quantified for HCPP and AC with a titration
method (Table S2).
In order to find out the sulfur content in s-AC, elemental analysis was carried out and 0.32
wt.% sulfur was confirmed in s-AC as shown in Table S3. Since no sulfur was found in AC,
all sulfur compounds in s-AC might be related to the sulfonic acids after the H
2
SO treatment.
4

Additionally, the TPD-MS spectra were obtained for AC and s-AC in Fig. S1. The TPD peak
with a mass signal (m/z= 48) originated from SO was observed at 250 ℃for s-AC during TPD
analysis (Fig. S1(a)). This also supports that the sulfonic acids were successfully incorporated
into the AC after the H SO treatment in the case of s-AC. Note that the sulfur content in c-s-
2
2
4
HCPP is far larger than that in s-AC (Table S3), which is closely related to the acid densities
of these supports.
The particle size of Pd for 2 wt.% Pd/c-HCPP, 2 wt.% Pd/c-s-HCPP, 1 wt.% Pd/s-AC, and
1
wt.% Pd/AC was estimated to be 7.1, 4.5, 5.2, and 2.5 nm, respectively, based on the TEM
images (Fig. S2).
3
.2. Partial oxidation of methane with H
2
and O
2
Table 1 shows the results of partial oxidation of methane at 0 ℃using FeSO
catalysts. No product and residual H was formed when Pd/HCPP was used as a catalyst for
generation of H from H and O (Table 1, Entry 1). However, higher concentrations of
residual H and turnover numbers (TONs) were obtained using Pd/c-HCPP and Pd/c-s-
HCPP in which acid-functionalized HCPP was used. This clearly implies that acid functional
groups in c-HCPP and c-s-HCPP are beneficial for H generation from H and O and that
in situ generated H is essential for methane oxidation using FeSO as a catalyst. When
Pd/AC and Pd/s-AC were used, the high TONs could be achieved but the selectivity to methane
oxygenates was too low due to overoxidation into CO . Alternatively, the presence of sulfuric
acid was beneficial in obtaining higher TONs and selectivity in methane oxygenates compared
to Pd/AC (Table 1, Entry 6). The addition of sulfuric acid can contribute to the stability of H
under acidic conditions, which is supported in our findings as seen by the higher concentration
of residual H in the presence of sulfuric acid (Table 1, Entry 6) compared with the absence
4
and various Pd
2
O
2
2
O
2
2
2
2
O
2
2
O
2
2
2
2
O
2
4
2
2
O
2
2
O
2

of any acid (Table 1, Entries 4 and 5). The separate experiment for H
was also carried out over the same catalysts. As shown in Table S4, the concentration of
formed H decreased in the following order: Pd/c-s-HCPP > Pd/c-HCPP >> Pd/AC ≈
Pd/HCPP ≈ Pd/s-AC. The promotional effect of acid on H synthesis was also confimed
Table S4, Entry 6).
When the partial oxidation of methane was carried out over FeSO
2
O
2
synthesis from H and
2
O
2
2
O
2
2
O
2
(
4
and Pd/c-s-HCPP in a
temperature range of 0 – 20 ℃, the yield of the total products increased but the selectivity to
methane oxygenates decreased with increasing reaction temperature (Table 2). When Fe-ZSM-
5
was used instead of FeSO
of residual H was 11 mM (Table 2). This implies that Fe-ZSM-5 is much less active than
FeSO with in situ generated H . Therefore, additional experiments were conducted to
determine the effect of the concentration of H on the catalytic activity for partial oxidation
of methane over Fe-ZSM-5. As shown in Table S5, the catalytic activity appears to be closely
related to the concentration of H : no product was obtained with 0.028 M H . However,
product yields were found with 0.28 M H . This implies that partial oxidation of methane
can proceed if the concentration of hydrogen peroxide is sufficiently high. In addition, the
effect of reaction temperature on methane oxidation was examined using H (0.028 M) over
4
, no product was obtained at 20 ℃even though the concentration
2
O
2
4
2
O
2
2
O
2
2
O
2
2
O
2
2
O
2
2
O
2
Fe-ZSM-5 in the temperature range of 10 − 70 ℃. As shown in Fig. S3, partial oxidation of
methane proceeded at temperatures in the range of 40 − 70 ℃, indicating that partial oxidation
of methane over Fe-ZSM-5 can be carried out at low temperatures with relatively high
concentrations of H
It is worth mentioning that the hydrogen peroxide efficiency, calculated as [moles of total
product]/[moles of H consumed], increased with the reaction temperature. These results
imply that the highly efficient Pd catalysts for in situ generation of H should be used at
relatively high temperatures in order to oxidize methane over the heterogeneous Fe-ZSM-5
2
O
2
or at relatively high temperature even with low concentrations of H
2
2
O .
2
O
2
2
O
2

catalyst. Consequently, the active Pd catalysts such as Pd/s-AC, Pd/c-HCPP, and Pd/c-s-HCPP
were examined for partial oxidation of methane over Fe-ZSM-5 in the presence of H and O
Noticeable amounts of methane oxygenates were obtained only when Pd/c-s-HCPP was used
as a catalyst for in situ generation of H (Table 3). When Pd/c-s-HCPP was used, the highest
2
2
.
2
O
2
-1
TOF, 3.8 h , was obtained at 30 ℃with a 94 % selectivity to methane oxygenates.
Considering all the methane oxygenates formed, the productivity of this catalytic system was
ca. 69 mmol gmetal(Fe) h-1 at 30 ℃, which is ranked one of the highest values reported (Table
-1
S6). On the other hand, no detectable methane oxygenates were found when Pd/s-AC was used
-
1
(
Table 3, Entry 1). This is in contrast with the results that TOF was approximately 46 h at 0
when homogeneous FeSO and Pd/s-AC catalysts were used (Table 1, Entry 5). This can be
due to the intrinsic difference in the reaction mechanism between homogeneous FeSO and
heterogeneous Fe-ZSM-5; FeSO is inactive for partial oxidation of methane with H in the
℃
4
4
4
2
O
2
absence of Pd catalyst [56], whereas Fe-ZSM-5 is active for this reaction with H
irrespective of the presence of Pd catalyst.
2
2
O ,
The total product yield increased when the reaction temperature was in the range of 30 – 50
(Fig. 1). The concentration of residual H decreased with increasing reaction temperature
Fig. 1, Table S7). Even though it is generally accepted that the H yield from H and O
decreased with an increase in temperature, the formed H can be efficiently utilized for
℃
2
O
2
(
2
O
2
2
2
2
O
2
methane oxidation, as revealed in this study (Fig. S3). In all cases, leaching of Fe species from
Fe-ZSM-5 catalysts was not observed based on ICP analysis.
The product distribution as a function of reaction time is presented in Fig. 2 (Table S8). As
the reaction time increases, the yield of methanol, formic acid, and CO
2
reach the final values.
This supports that partial oxidation of methane is a consecutive reaction from methane to CO
2

through methanol and formic acid. Based on these data, the optimum reaction time is 4 h under
given reaction conditions. The decomposition of H was only observed after 4 h without any
additional product being produced. This indicates that the role of Pd catalyst for generating
high concentrations of H is essential for this catalyst system.
2
O
2
2
O
2
4
. Conclusions
In summary, we demonstrated the partial oxidation of methane in water over a combination
of Fe-ZSM-5 and Pd/c-s-HCPP catalysts in the presence of H and O . The acid-functionalized
HCPP was essential for in situ generation of H over Pd catalyst in the absence of an added
2
2
2
O
2
acid. Metal leaching of Fe-ZSM-5 was not observed due to the absence of liquid acid in the
reaction system. The cooperation of Pd/c-s-HCPP and Fe-ZSM-5 obtained a TOF of 3.8 h-1
and selectivity to methane oxygenates of 94 % at 30 ℃.
Credit author statement
Jongkyu Kang: Validation, Investigation, Data Curation, Writing - Original Draft. Pillaiyar
Puthiaraj: Investigation, Data Curation. Wha-seung Ahn: Writing - Review & Editing. Eun
Duck Park: Conceptualization, Writing - Review & Editing, Supervision, Project
administration, Funding acquisition.
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
Acknowledgment

This work was supported by the C1 Gas Refinery Program through the National Research
Foundation of Korea (NRF) funded by the Ministry of Science and ICT
(2015M3D3A1A01064899).
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Scheme 1. The partial oxidation of methane over Fe-ZSM-5 and Pd catalyst with H
generated in situ from H and O
2
O
2
2
2
.

Fig. 1. The effect of reaction temperature on product profile after partial oxidation of methane over Fe-ZSM-5 and
Pd/c-s-HCPP using H and O . Reaction conditions: 50 mg of 0.919 wt.% Fe-ZSM-5 and 25 mg of 2 wt.% Pd/c-s-
HCPP. Reaction time= 1 h. Vliquid= 30 mL, H O. PCH4= 15 bar, PH2= 3 bar, PAir= 10 bar.
2
2
2

Fig. 2. The effect of reaction time on product profile after partial oxidation of methane over Fe-ZSM-5 and Pd/c-s-HCPP
using H and O . Reaction conditions: 50 mg of 0.919 wt.% Fe-ZSM-5 and 25 mg of 2 wt.% Pd/c-s-HCPP. Reaction
temperature = 40 ℃, Vliquid = 30 mL, H O. PCH4 = 15 bar, PH2= 3 bar, PAir = 10 bar.
2
2
2

Table 1. Catalytic performance for partial oxidation of methane over FeSO
4
and various Pd catalysts in the presence of O
2
and H
.a
2
Average particle
H
2
O
2
Product (μmol)
Selectivity to
oxygenates
b
c
TONd
Entry
Pd catalyst
size of Pd
remaining
(mM)
CH
3
OH
HCOOH
CH
3
OOH
CO
2
(
nm)
(%)
1
2
3
4
5
3.5 % Pd/HCPP
2.6 % Pd/c-HCPP
4.9 % Pd/c-s-HCPP
1 % Pd/AC
12
0
0
4
0
7
0
3
4
2
2
8
Trace
5
0
~0
1.7
5.2
3.0
2.5
5.2
2.5
9
74
82
31
25
87
15
1.4
2.0
17
7
16
15
18
201
6
3.0
11
9
62
88
34
7.9
1 % Pd/s-AC
10.5
6
1 % Pd/ACe
13
22.9
a
Reaction conditions: [FeSO
4
]= 0.37 mM and Pd catalyst (50 mg for Pd/ACs, 5 mg for Pd/HCPPs), reaction temperature: 0 ℃, reaction time: 30 min. Vliquid= 30 mL, H
2
O. PCH4= 15 bar, PH2= 3
bar, Pair= 10 bar;
b
Average particle size of Pd for each Pd catalyst was adapted from the previous report [45] or determined using TEM images (Fig. S2)
c
d
e
Selectivity to methane oxygenates was calculated as [moles of methane oxygenates excluding CO
Turnover number was calculated as [moles of total products]/[moles of Fe introduced];
2
]/[moles of total products] x 100 (%);
1
2 4 2
5 mM of aqueous H SO solution was used instead of H O.

Table 2. Catalytic performance for partial oxidation of methane over FeSO4 or 0.919 wt.% Fe/ZSM-5 with 4.9 wt.% Pd/c-s-HCPP in the presence of H and
2
a
O
2
.
Product (μmol)
Selectivity, %)
Selectivity to
methane
oxygenates
(
Temp.
TOFd
Fe
H
2
O
2
remaining
(mM)
Fe catalyst
TONc
b
-1
(
μmol)
(
℃)
(h )
CH
3
OH
HCOOH
CH
3
OOH
CO
2
(
%)
FeSO
FeSO
FeSO
4
4
4
11.2
11.2
11.2
8.2
0
15
21
21
11
7(21)
3(8)
16(48)
17(44)
20(41)
0(0)
4(12)
3(8)
2(4)
0(0)
6(18)
16(41)
21(43)
0(0)
82
59
57
0
3.0
3.5
4.4
0
6.0
7.0
8.8
0.0
10
20
20
6(12)
0(0)
0
.919%Fe-ZSM-5
a
Reaction condition: 4.9 wt.% Pd/c-s-HCPP with Fe catalyst was used. reaction time= 30 min. Vliquid= 30 mL, H
2
O. PCH4 =15 bar PH2= 3 bar PAir= 10 bar.
b
c
d
The selectivity to methane oxygenates was calculated as [moles of methane oxygenates excluding CO
TON was calculated as [moles of total products]/[moles of Fe introduced].
TOF was calculated as TON/[reaction time (h)].
2
]/[moles of total products] x 100 (%).

Table 3. Catalytic performance for partial oxidation of methane over 0.919 wt.% Fe-ZSM-5 and
a
various Pd catalyst in the presence of H
2
and O .
2
Selectivi
ty to
Product (μmol)
Average
Reaction
H
2 2
O
d
Entr
y
particle
remaini
ng
(mM)
TOF
Pd catalyst
time
oxygena
b
-1
size of Pd
HCOO
H
CH
3
OO
(h )
c
(
h)
CH
3
OH
CO
2
tes
(
nm)
5.2
7.1
4.5
H
0
0
0
(
%)
1
2
2
% Pd/s-
AC
1
2
3
4
2
2
0
0
0
2
0
0
1
1
4
0
0.03
0.2
% Pd/c-
HCPP
61
94
% Pd/c-
27
17
42
3.8
s-HCPP
4
.9%
4
Pd/c-s-
HCPP
3.0
2
0.4
4
0
0
0
100
0.5
a
Reaction conditions: 0.919 wt.% Fe-ZSM-5 50 mg, 1 wt.% Pd/s-AC 50 mg, 2 wt.%Pd/HCPPs 25 mg, or 4.9 wt.% Pd/c-s-
HCPP 10 mg. Reaction temperature= 30 ℃. Vliquid= 30 mL, H O. PCH4 =15 bar, PH2= 3 bar, PAir= 10 bar
TEM images were used to estimate the average particle size of Pd (Fig. S2)
2
b
c
2
Selectivity to methane oxygenates was calculated as [moles of methane oxygenates excluding CO ]/[moles of total
products] x 100 (%)
d
Turnover frequency was calculated as [moles of total products]/[(moles of Fe introduced)x(reaction time (h))].
2
4