Direct oxidative carbonylation of methane to acetic acid via high-valent iron-oxo mediated water activation

Domain: chemistry
Published: 2026-03-07
Editorial outcome: minor_revision

Abstract
Direct conversion of CH into value-added chemicals is impeded by the inert C-H bonds and
4
inefficient C-C coupling. We report a spatially separated Rh-O-Fe active-site architecture that
decouples CH and H O activation through a high-valent-metal mediated radical mechanism,
4 2
enabling selective CH COOH synthesis. In-situ infrared, operando Mössbauer spectroscopy, and
3
quasi in-situ high-field EPR reveal that O oxidizes Rh and Fe to high valence states. Rh(III)
2
activates CH to •CH , while Fe(IV)=O dissociates H O into •OH through a truncated water-gas
4 3 2
shift pathway. •OH rapidly reacts with CO to form •COOH intermediates, which couples with
•CH within the zeolite to yield CH COOH. This dual-site strategy circumvents kinetic limits of
3 3
conventional water-gas shift and CO insertion steps. The catalyst achieves 18.2 mmol g -1 h-1
cat
CH COOH with 92% selectivity and 100-hour stability in continuous operation. This study
3
establishes radical decoupling enabled by high-valent metal sites as a design principle for selective
alkane oxidation.

Introduction
Methane (CH
4
), the primary component of natural gas, stands as a critical yet underutilized
carbon resource due to the formidable challenges in its selective activation and functionalization
1-3. The direct conversion of CH into C products demand overcoming two intrinsic barriers: the
4 2+
high bond dissociation energy of sp3 C-H bonds (439 kJ mol-1) and the sluggish kinetics of C-C
bond formation 4-6. Acetic acid (CH COOH), a cornerstone chemical with an annual demand of 18
3
million metric tons 7,8, exemplifies this challenge, as its industrial production still relies on an
indirect syngas-derived methanol carbonylation process plagued by energy intensity 9-12. The one-
step methane oxidation process for CH COOH production enables efficient synthesis through
3
direct CH activation under mild conditions, bypassing the complex steps required in traditional
4
methanol carbonylation processes, such as methanol synthesis and the use of halogen promoters,
thus demonstrating significant advantages in feedstock economy and process simplicity. Although
this new pathway still requires further refinement, it has already provided a more atom efficient
and sustainable alternative for the green production of CH COOH.
3
Current strategies for direct CH oxidative still faces several critical limitations. For C-H
4
activation, conventional strategies typically rely on unstable zero-valent metals (e.g., Rh0) or
external addition of H O to generate •OH 13. In some cases, the complete water-gas shift (WGS)
2 2
cycle (CO + H O → CO + H ) is involved to in-situ produce H O 14-16, which then undergoes
2 2 2 2 2
multiple steps to form •OH for CH activation. This multistep pathway not only prolongs reaction
4
process but with progressively declining energy efficiency, significantly limiting the overall
catalytic performance. For C-C coupling with CO, a kinetically sluggish process remains the rate-
determining bottleneck. For example, the methanol carbonylation proceeds efficiently through a
migratory insertion of the neutral covalent ligands •CH and *CO at a metal center, facilitated by
3
CH I, enabling high-efficiency carbonylation 17. Therefore, in the direct CH carbonylation, the
3 4
in-situ generated •CH do not readily couple with neutral CO 18. Although recent efforts have
3
focused on enhancing •OH and •CH
3
concentrations or optimizing active sites 19,20, the critical C-
C coupling step (involving •CH
3
-CO vs. •CH
3
-•COOH) in this pathway has never been
experimentally verified, hindering the development of efficient catalytic systems.
To overcome the kinetic and mechanistic constraints of direct CH carbonylation, we report
4
a catalyst design that integrates C-H activation with spatially orchestrated radical coupling. By
constructing Rh-O-Fe active sites within ZSM-5, the RhFe/ZSM-5 catalyst leverages a CO-
assisted water activation mechanism that circumvents the inefficiencies of conventional WGS
cycles. Here, Rh(III) can active CH
4
to generate •CH
3
, while O
2
treatment sustains the dynamic
valence transition of Rh(III), and simultaneously oxidizes Fe sites to form Fe(IV)=O species, this
high-valent Fe-oxo entity facilitates direct cleavage of H O into •OH, which rapidly react with CO
2
to produce •COOH intermediates. These subsequently couple with Rh-derived •CH species within
3
the confined micropores of the zeolite to form CH COOH. This integrated mechanism bridges the
3
long-standing gap between CH activation and selective C-C bond coupling, providing a new
4
paradigm for efficient and scalable natural gas valorization.

Results
Catalytic performance of CH carbonylation into acetic acid
4
Monometallic ZSM-5-based catalysts with 0.6 wt% metal loading (Rh, Fe, Ni, Co, Cu, Zn,
Ag, Pd, Au, Re, Pt, Mo) were synthesized via a template-free seeded growth method and evaluated
in a batch reactor. Among these, Rh/ZSM-5 exhibited the highest catalytic performance, achieving
a CH COOH yield of 3.2 mmol g -1 h-1 with a selectivity of 61% at 463 K, as determined by GC
3 cat
and 1H NMR (Supplementary Figure 1-2). Notably, the Fe monometallic has little reactivity. In
contrast, introducing Fe into Rh/ZSM-5 to form RhFe/ZSM-5 significantly improved catalytic
performance, yielding 18.2 mmol g -1 h-1 of CH COOH with a selectivity of 91.8% at 463 K
cat 3
under optimized reaction conditions (Table 1 entry 1, Supplementary Figure 3-4). The
corresponding TOF reached ~216 h-1, which is ~2.5 times that of monometallic Rh/ZSM-5 (92 h-
1) (Supplementary Figure 5). It is worth mentioning that the reaction was initiated at 363 K (Table
1 entry 2). This performance was significantly higher than AuFe/ZSM-5, PdFe/ZSM-5,
RuFe/ZSM-5, PtFe/ZSM-5, RhNi/ZSM-5, RhCu/ZSM-5, and different supports, as well as
previously reported catalysts in the literature (Figure 1a, Supplementary Figure 6-7, and
Supplementary Table 1).
Furthermore, when Rh/ZSM-5 and Fe/ZSM-5 catalysts were physically mixed and tested
under identical reaction conditions, no significant improvement in CH
3
COOH yield or selectivity
was observed compared with Rh/ZSM-5 (Supplementary Figure 2b), highlighting the crucial role
of the synergistic interaction between Rh and Fe in RhFe/ZSM-5. The lower activity of the
spatially separated RhFe/ZSM‑I catalyst further confirms that Rh-Fe synergy is essential for high
performance. Highly active Rh-Fe sites not only enhance CH ’s C-H bond activation but also
4
modify the reaction pathway of CH oxidative coupling. Additionally, a systematic investigation
4
of Fe content, while maintaining Rh loading at 0.6 wt.%, revealed a gradual increase in both
CH COOH yield and selectivity (Supplementary Figure 8), further demonstrating that the Fe
3
introduction plays a key role in optimizing the reaction mechanism.
For comparison, a catalyst was synthesized using a commercial ZSM-5 (Si:Al=18) support
via an impregnation method (denoted as RhFe/ZSM-5-C), and exhibited a significantly lower
CH
3
COOH yields of 5.08 mmol g
cat
-1 h-1 with a selectivity of 55.5% (Supplementary Figure 9).
BET surface area analysis revealed similar values for ZSM-5-C (373 m2/g) and ZSM-5 (338 m2/g),
indicating that the differences in catalytic performance stem primarily from variations in metal
active sites or acidity rather than textural properties (Supplementary Figure 10).
CH COOH formation was observed only when CH , CO, and O were simultaneously present,
3 4 2
as no detectable products were detected in control experiments omitting any of these components
(Table 1, Entry 3-5). These findings demonstrate that CH , CO, O , and H O are all essential for
4 2 2
CH COOH production. Additionally, when H was used as a substitute for CO, only C oxygenates,
3 2 1
such as CH OH, HCOOH, were detected (Supplementary Figure 11a, Supplementary Table 2
3
Entry 1). This suggests that CH COOH is produced through the C-C coupling of CH and CO,
3 4
rather than from environmental contaminants or the catalyst itself. In addition, comparative
experiments performed with CH COOH in the presence and absence of CO confirmed that CO is
3 2
predominantly formed through CO oxidation (Supplementary Figure 11b). Under standard
methane oxidation conditions, the rate of CO
2
production reached 41.6 mmol g
cat
-1 h-1, surpassing
that from CH COOH. For clarity, both liquid-phase selectivity (>92% toward CH COOH among
3 3
oxygenated products) and total carbon selectivity (including CO , 42.3% for CH COOH) are
2 3
provided (Supplementary Figure 12-13). These results indicate that, although the catalyst shows
high selectivity to acetic acid in the liquid phase, the oxidation of CO to CO represents a major
2
competing pathway and warrants further optimization in future studies.
To confirm the heterogeneous nature and stability of the catalyst, further control experiments
were conducted 21. In the absence of catalyst, as well as in the presence of FeCl , Rh(NO ) , or a
2 3 3
mixture of these species, no catalytic activity was observed (Supplementary Table 2 Entry 2-5).
Hot filtration experiments further confirmed the heterogeneous nature of the catalytic process, as
no additional reaction occurred after 2 hours (Figure 1b). The stability of RhFe/ZSM-5 was
evaluated over five reaction cycles, showing negligible leaching of Rh and Fe species, as
determined by ICP analysis (Supplementary Table 3). In addition, batch reactions conducted under
lower CH
4
partial pressure in air and CO achieved a CH
4
conversion of up to 24.8% within 3 h
(Figure 1c), with > 98% selectivity to CH COOH. Even when prolonging reaction time to 6 h, no
3
methanol and formic acid were not detected by 1H-NMR (Supplementary Figure 14). Furthermore,
this reaction system is effective not only for CH
4
but also for ethane, which yield acids with an
additional carbon atom (Supplementary Figure 15).
Identification of active sites
The synergy observed by the presence of both Rh and Fe in the CH reaction to CH COOH
4 3
over the prepared ZSM-5-based catalyst required careful study to elucidate a structure-activity
relationship. XRD analysis shows that the ZSM-5 crystal structure was well preserved in Fe/ZSM-
5, Rh/ZSM-5, and RhFe/ZSM-5 22, with no characteristic diffraction peaks corresponding to Rh
and Fe metals (Supplementary Figure 16). The isolated Fe and RhO nanoparticles species were
detected in the ZSM-5 as confirmed by the annular dark-field scanning transmission electron
microscopy (AC-HAADF-STEM) and EDX (Figure 2a-b, Supplementary Figure 17-18). In the
UV-vis spectrum (Supplementary Figure 1 9), no obvious absorption at >400 nm was observed,
which implies that homogeneous isolated Fe species was confined in the micropores 3,23.
X-ray absorption fine structure spectroscopy (XAS) was used to systematically study
electronic structures and coordination environments of as-prepared catalysts. In the Fourier
transform (FT) k3-weighted extended EXAFS spectrum, the absence of Fe-Fe shell verifies that
the Fe atoms were atomically dispersed in the Fe/ZSM-5 and RhFe/ZSM-5 (Figure 2c), Rh exhibits
a RhO curve similar to that of Rh O (Figure 2d). For Rh/ZSM-5, the best fitting of the Rh-O
2 3
backscattering path reveals the Rh-O coordination numbers of 6 in the first coordination shell
(Supplementary Figure 20, Supplementary Table 4). For Fe/ZSM-5, the major peak at ∼1.50 Å in
Fe K-edge EXAFS spectrum could be attributed to the Fe-O configurations with a coordination
number of 4. For RhFe/ZSM-5, a primary peak at 1.52 Å could be attributed to Rh-O shell and
Rh-O-Fe in second shell in 2.42 Å was observed (Figure 2d), which is longer than that of Fe-O-Fe
in Fe/ZSM-5 (2.34 Å), which suggests the presence of an Rh-O-Fe structure in RhFe/ZSM-5.
To confirm the Rh-O-Fe structure, 57Fe Mössbauer spectra of Fe/ZSM-5 and RhFe/ZSM-5
were collected at 4.2 K using a 57Co(Rh) source (Figure 2e-f, Supplementary Table 5). The spectra
were fitted with a doublet (assigned to dimeric high-spin Fe(III)-Fe(III) species) and a broad sextet
(assigned to isolated Fe(III) ions). Minor isolated Fe(II) species (2-3%) were also detected. The Fe(III)
paramagnetic doublet at 4.2 K indicates well-distributed Fe species without sufficient neighboring
Fe atoms to generate a magnetic field, even at cryogenic temperatures. The doublet suggests spin
relaxation (spin-flipping) due to coupling between at least two neighboring Fe atoms, supporting
a dimeric structure. When Fe(III) ions are spaced >1.5 nm apart, slow spin-spin relaxation results in
paramagnetic hyperfine splitting. The broad sextets correspond to isolated Fe(III) ions with large
hyperfine fields (>55 T), as shown in Supplementary Figure 21, with a maximum magnetic field
distribution at ~56 T. Typical Fe oxide clusters (~54 T) are likely absent. Paramagnetic hyperfine
splitting in Fe-ZSM-5 aligns with prior observations 24. Isomer shifts for Fe(III) species suggest
octahedral extra-framework coordination, as tetrahedral positions would exhibit smaller shifts
(~0.4 mm/s). The Rh-Fe sample shows more dimeric species (28%) than the Fe-only sample (18%),
potentially indicating Fe-Rh dimer formation, consistent with XAS results.
We further investigated the interaction between Fe and Rh through electronic structure
analysis. The Rh K-edge XANES spectrum (Supplementary Figure 22a) showed that the
introduction of Fe into Rh/ZSM-5 increased the oxidation state of Rh in RhFe/ZSM-5 when
compared to that in Rh/ZSM-5. Similarly, Fe K-edge XANES spectrum (Supplementary Figure
22b) revealed that Fe in RhFe/ZSM-5 has a lower oxidation state than in Fe/ZSM-5, indicating
electron transfer from Rh to Fe. The oxidation state of Fe is between +2 and +3, while Rh is below
+3. This charge redistribution was further supported by the Rh 3d XPS spectrum (Supplementary
Figure 23), where the characteristic peaks at 312.5 eV and 307.7 eV in Rh/ZSM-5 shift to 312.9
eV and 308.2 eV in RhFe/ZSM-5, suggesting an increased electron density on Fe in RhFe/ZSM-
5.
To further elucidate the unique electronic structure of the catalyst, we carried out in‑situ
CO‑DRIFTS measurements (Supplementary Figure 24). ZSM‑5 and monometallic Fe/ZSM‑5
show characteristic peaks of gaseous CO at 2171 cm-1 and 2118 cm-1, whereas Rh/ZSM‑5 exhibits
linear CO adsorption bands at 2107 cm-1 and 2040 cm-1. Notably, RhFe/ZSM‑5 catalyst shows a
distinct blue shift to 2104 cm-1 and 2037 cm-1, indicating a decrease in the electron density of Rh
sites. This directly reflects the electronic interaction between Rh and Fe, specifically, electron
transfer from Rh to Fe, and is fully consistent with XPS and XANES data. In addition, the
appearance of a bridged CO adsorption peak at 1860 cm-1 further confirms the presence of Rh
nanoparticles, in agreement with HAADF‑STEM observations.
To directly investigate the influence of acidic sites on catalytic performance, we prepared a
series of RhFe/ZSM-5-X catalysts with identical metal loadings but varying SiO /Al O ratios (X
2 2 3
= 18, 25, 40, 60, 80). NH -TPD and pyridine adsorption infrared spectroscopy (Py-IR) results show
3
that the acidity of the catalysts significantly decreases with increasing SiO /Al O ratios
2 2 3
(Supplementary Figure 25a-b). Catalytic performance tests (Supplementary Figure 25c) indicate
that both the yield and selectivity of CH COOH decrease markedly as the support SiO /Al O
3 2 2 3
increases. Among these, RhFe/ZSM-5-18 exhibits the best CH COOH performance (18.2 mmol
3
g
cat
-1 h-1), while RhFe/ZSM-5-80 shows a substantial decline in activity (0.4 mmol g
cat
-1 h-1). This
clearly demonstrates that zeolite acidity is a key factor in maintaining high catalytic performance.
To further differentiate the acidic contribution, Na+ ion exchange was performed on
RhFe/ZSM-5 (SiO /Al O =18) to obtain RhFe/Na-ZSM-5. Py-IR reveals a significant decrease in
2 2 3
both Brønsted and Lewis acid sites, confirming effective neutralization of the acidic sites
(Supplementary Figure 26a). Correspondingly, the CH COOH yield over RhFe/Na-ZSM-5
3
decreases to 3.8 mmol g -1 h-1, further proving that acidic sites (particularly exchangeable H+) are
cat
indispensable for high catalytic activity (Supplementary Figure 26b). In CO-TPD (Supplementary
Figure 26c), RhFe/ZSM-5 exhibits a strong CO desorption peak at 665 K, indicating its strong
adsorption capacity for CO. In contrast, the desorption peak for RhFe/Na-ZSM-5 shifts to 553 K
with reduced intensity, demonstrating that Na⁺ exchange not only weakens acidity but also the CO
adsorption strength of catalyst.
Comparison with Conventional WGS and Key Steps
Previous studies have proposed that water in the Water-Gas Shift (WGS) reaction plays a
crucial role in CH activation 14,25, as H generated from WGS facilitates the conversion of O into
4 2 2
H O , which subsequently produces •OH species for CH activation. However, this multi-step
2 2 4
process introduces significant kinetic limitations that hinder overall reaction efficiency. In contrast,
our RhFe/ZSM-5 catalytic system appears to follow a fundamentally different pathway.
Isotope kinetic studies in the WGS system (CO and H O) catalyzed by RhFe/ZSM-5 showed
2
a kinetic isotope effect (KIE) ratio of K /K (H ) = 4 at 463 K (Supplementary Figure 27),
H D 2
confirming that H O decomposition is the rate-determining step (RDS) in the WGS reaction.
2
However, in the CH , CO, O , and H O system, the KIE ratio for CH COOH formation over
4 2 2 3
Rh/ZSM-5 was approximately 1 (Figure 3a, Supplementary Figure 28), whereas RhFe/ZSM-5
exhibited an inverse KIE (K /K = 0.21) (Figure 3b, Supplementary Figure 29-30), suggesting
H D
that the presence of Fe promotes H O activation, thereby eliminating water activation as the rate-
2
determining step in the reaction.
Although a low concentration of H was detected during WGS over RhFe/ZSM-5
2
(Supplementary Figure 31), the significant difference between the formation rates of H
2
and liquid
products suggests that H
2
does not play a major role in CH
3
COOH formation. This conclusion is
further corroborated by the effect of H addition on product selectivity, as the introduction of H
2 2
resulted in only a 1.5 increase in CH COOH yield, whereas methanol yield increased by 3.7
3
(Supplementary Figure 32). In the in-situ DRIFTS measurements of CO, O
2
, and H
2
O reaction
system (Figure 3c and 3d), two obvious signals at 950 cm-1 and 932 cm-1 were observed on
Rh/ZSM-5 and RhFe/ZSM-5, which can be assigned to side-on O adsorption and O-O stretching
2
of surface peroxo species 26-28. Moreover, a new peak at 840 cm-1 and a shoulder at 864 cm-1 were
detected on Rh/ZSM-5 (Figure 3c), attributed to the O-O stretching of adsorbed and free H O
2 2
molecules, respectively 28. Nevertheless, these two signals were absent on RhFe/ZSM-5 (Figure
3d), indicating that H O wasn’t generated on RhFe/ZSM-5.
2 2
Identification of Active Species and Oxygen Sources
In order to study the influence of •OH, excess H O instead of O was added into the system of
2 2 2
CH , CO, and H O (Supplementary Figure 33). Both the yield and selectivity of CH COOH were
4 2 3
lower than those observed in the CH , CO, O , and H O reaction system. Notably, although the
4 2 2
concentration of •OH radicals in the H O system was significantly higher than in the CO and O
2 2 2
systems (Supplementary Figure 34), the CH COOH yield remained relatively low, indicating that
3
•OH radicals can participate in CH COOH formation, but not the primary pathway of CH
3 4
activation. These findings demonstrate that an alternative activation mechanism must be
responsible for CH conversion rather than H +O →H O →•OH pathway.
4 2 2 2 2
In the isotope-labeling experiments of CH , CO, 16O , and H 18O (Figure 3e, Supplementary
4 2 2
Figure 35), isotopically labeled CH CO18OH was detected, whereas only CH CO16OH was
3 3
observed when CH , CO, 18O , and H 16O were used. These results confirm that the hydroxyl
4 2 2
oxygen in CH COOH originates from H O decomposition rather than O . Additionally, when using
3 2 2
CH , CO, O and D O as reactants, CH COOD was detected, demonstrating that the hydrogen in
4 2 2 3
the hydroxyl group of CH COOH originates from H O.
3 2
Reaction Intermediates and Coupling Mechanism
To explore the intermediates after introducing Fe active sites in this reaction system, we carried
out in-situ Mössbauer spectroscopy analysis 29 (Figure 4a-b and Supplementary Table 6-7) and
high-frequency (240 Hz) quasi in-situ electron paramagnetic resonance (HF-EPR) spectroscopy
analysis (Figure 4c) 30,31. In the in-situ Mössbauer measurements, both the source and the samples
were cooled to 4.2 K after undergoing reaction at 463 K. In this way, the data can directly be
compared to data obtained at room temperature, since both source and absorbers were maintained
at equal temperatures, thereby experiencing similar second-order Doppler shifts.
After an initial O /Ar treatment (Figure 4a-b, Treatment A), the presence of Fe(IV)=O species
2
were clearly in the sub-Mössbauer-spectra of Fe/ZSM-5 and RhFe/ZSM-5 (Supplementary Table
6-7), having low isomer shift values, in the 0.14-0.21 mm/s range (Fe(IV)=O (I) and Fe(IV)=O (II),
represent O=Fe(IV)-O-Fe(IV)=O, and highly dispersed Fe(IV)=O, respectively, indicating O
2
facilitates the Fe(IV)=O formation 32-36. Additionally, the presence of a doublet signal at 4.2 K
indicates the spin relaxation effects in highly dispersed species-the doublet (cyan) with 0.21 mm/s
isomer shift being assigned to diiron (IV) complexes, O=Fe(IV)-O-Fe(IV)=O, in the Fe/ZSM-5 37.
Simultaneously, the observation of a well-defined sextuplet signal at 4.2 K (Figure 4a) (magenta
sub-spectrum) with such highly dispersed species, indicates the presence of superexchange
interactions over long-range, mediated by the oxygen atoms 38. The superexchange interactions
change the spin state at the oxoiron (IV) sites 39, as evidenced by the lower measured isomer shift
values (0.14-0.16 mm/s), suggesting the presence of high-spin Fe(IV) species. This result is
consistent with the high-field EPR (HF-EPR) spectra (15 K, 240 Hz) (Figure 4c Treatment A) after
12 h O treatment at 463 K, where the disappearance of Fe and Rh signals coincided with the
2
oxidation of Fe to Fe(IV) (S=2) and Rh to Rh(III)(S=0) (Supplementary Figure 36).
The second dimeric doublet (Figure 4a, red colour) in the Mössbauer spectra measured after the
O /Ar treatment has an isomer shift value (0.31 mm/s), which is smaller than that measured in the
2
fresh samples (0.53/0.50 mm/s minus 0.14 mm/s correction). This could indicate the formation of
mixed dimeric complexes like HO-Fe(III)-O-Fe(IV)=O in Fe/ZSM-5 39. This component is higher in
the Rh-containing sample (43 vs. 30 %), due to the partial formation of mixed Rh-O-Fe species.
Also, the quadrupole splitting values are relatively different in these dimeric doublet signals (1.44
vs. 1.50 mm/s), indicating higher charge asymmetry around the Fe atoms in the Rh-containing
zeolite. Under the O
2
/Ar reaction conditions, the content of Fe(III)-D in RhFe-ZSM-5 (43%) is
higher than that in Fe-ZSM-5 (30%). This is due to Rh’s ability to donate electrons to Fe. XPS and
EXAFS analysis confirm this electron transfer process, further indicating that HO-Fe(III)-O-Rh
species constitute a significant proportion of Fe(III)-D in Fe(Rh)-ZSM-5.
Upon CO introduction (Figure 4a and b, Treatment B), the signal of dimeric iron Fe(III)-D
decreased in the Mössbauer spectra, while the signal of dispersed iron Fe(III)-PHS (blue) increased.
This suggests that CO preferentially reacts with the bridging oxygen in HO-Fe(III)-O-Rh(Fe) dimers
rather than Fe(IV)=O (I) and Fe(III)-D. The selective reaction of CO with the bridging oxygen
disrupts long-range superexchange interactions mediated by oxygen atoms, leading to the
conversion of Fe(III)-D into Fe(III)-PHS. The amount of Fe(IV) showed little change, suggesting its
low reactivity in oxidizing CO, which aligns well with the EPR results (Figure 4c, treatment B).
When CH was introduced (Figure 4a and b, Treatment C), the spectral contributions of Fe(IV)
4
species remained largely unchanged of RhFe/ZSM-5 in Mössbauer spectroscopy, but their
hyperfine parameters of Fe(IV) shifted more significantly than that Fe/ZSM-5, suggesting that the
neighboring Rh sites play a crucial role in C-H activation, rather than the Fe(IV)=O species. Upon
CH introduction to the RhFe/ZSM system post O /CO treatment, the HF-EPR signal re-emerged
4 2
(Figure 4c Treatment C), demonstrating that CH can be activated by high-valent metals (Fe(IV) or
4
Rh(III)). Importantly, the Mössbauer spectra revealed no significant change in Fe(IV) species after
CH exposure, confirming that the observed signal in the HF-EPR originates from the reduction
4
of Rh(III) to Rh(II), while Rh(III) actively participated in CH activation.
4
Subsequent H O treatments (Figure 4a and b, Treatment D) appear to reset the samples to their
2
fresh state. However, the dimeric complex in the RhFe/ZSM-5 remained with low isomer shift
values (0.32-0.34 mm/s), indicating the continued presence of mixed Fe(III)-O-Rh species. The Rh-
O -H O system (Supplementary Figure 37) also exhibited a Rh(II) signal, proving that both Fe(IV)
2 2
and Rh(III) can activate water. Notably, the signal amplitude increased markedly upon H O addition,
2
indicating that high-valent metals extract •H radicals (not protons) from H O, concurrently
2
generating •OH radicals.
Furthermore, in-situ infrared spectroscopy was employed to capture the key intermediates in
CH
4
activation and coupling (Supplementary Figure 38). Water vapor was introduced into the in-
situ cell using an N
2
carrier gas, and a baseline was recorded after stabilization. Sequential addition
of CH and CO yielded no significant signals corresponding to products such as *C=O in
4
CH COOH. However, upon O introduction, the spectrum underwent significant changes,
3 2
revealing signals for •CH
3
and *C=O at 1467 cm-1 and 1743 cm-1, respectively 40,41. Notably, the
water consumption peak at 3670 cm-1, coinciding with the results of Mössbauer and HF-EPR. EPR
further detected the presence of •CH radicals (Supplementary Figure 39). In the in-situ FTIR
3
spectra under condition of O /H O and O /H O /CO over Fe/ZSM-5 (Supplementary Figure 40),
2 2 2 2
the vibration of CO was observed in the range of 2011-2256 cm-1, while the characteristic peaks
at 1576 cm-1 and 1352 cm-1 correspond to the *COOH, indicating CO hydroxylation 42. Compared
with the O /H O/CH system (Supplementary Figure 41), O /CO/H O/CH system shows
2 2 4 2 2 4
significantly enhanced HF-EPR signals, demonstrating that CO promotes the conversion of H O
2
and CH into •OH and •CH radicals, facilitating •COOH formation via •OH/CO coupling and
4 3
subsequent CH COOH synthesis.
3
To further confirm the C-C coupling pathway, we conducted control experiments using
methanol, formaldehyde, and formic acid as potential reactants under identical conditions.
However, none of these compounds resulted in the formation of CH COOH (Supplementary Table
3
2, Entries 6-8), effectively ruling out a methanol-based pathway, such as the Monsanto process.
When CH I was used as a substitute for CH , an increase in the yields of both CH COOH and
3 4 3
CH OH was observed, indicating the involvement of •CH radicals in the reaction (Supplementary
3 3
Figure 42).
Complementary DFT calculations further support a radical based pathway involving •CH and
3
•COOH coupling, which exhibits a significantly lower energy barrier than alternative routes.
Specifically, two plausible coupling mechanisms were assessed: (i) *CH coupling with *CO,
3
followed by hydroxylation, and (ii) *CH coupling directly with *COOH formed via *CO and H O
3 2
activation. The former pathway faces a prohibitive barrier of 1.52 eV (Figure 5a), whereas the
latter proceeds with a much lower barrier of 0.48 eV, aligning well with the experimental
observation of *COOH intermediates. The overall reaction mechanism is summarized in Figure
5b, Supplementary Figure 43, and Supplementary Table 8. Together, these results highlight
*COOH mediated C-C coupling as the energetically and mechanistically preferred carbonylation
route.
Catalyst Performance and Stability
Through mechanistic insights into CH oxidation, we demonstrate that supported RhFe/ZSM-
4
5 catalysts exhibit exceptional stability in acetate acid production during batch reactions
(Supplementary Figure 44). This performance was successfully translated to continuous-flow
operation using RhFe/ZSM-5 in CH oxidative carbonylation (Supplementary Figure 45). In the
4
flow reactor at 503 K (Figure 5c), the RhFe/ZSM-5 catalyst maintained stable activity for 100
hours, achieving a consistent CH COOH productivity of 0.71 mmol g -1 h-1 with 83% selectivity
3 cat
and no observable deactivation. Meanwhile, the Rh/ZSM-5 catalyst showed significantly lower
performance, yielding only 0.14 mmol g -1 h-1 with 68% CH COOH selectivity (Supplementary
cat 3
Figure 46). Comprehensive post-reaction of batch and continuous characterization revealed
preserved crystallinity and valence state by XRD and XPS (Supplementary Figure 47-48), and
STEM confirmed maintained dispersion of both noble metal nanoparticles and atomic Fe sites all
structural features that directly account for the outstanding catalytic stability observed
(Supplementary Figure 49-50).

Methods
Materials and Chemicals. Ferrous chloride (FeCl , AR, 99%, Macklin), Rhodium nitrate solution (Rh(NO ) ,
2 3 3
AR, 5% in H O, Macklin), and other metal chlorides (Cu, Au, Pd) (AR, 99%, Macklin) were used as metal
2
precursors. Silica solution (SiO , 29%-31%, Macklin), Sodium hydroxide (NaOH, AR, Macklin), sodium
2
aluminate (NaAlO , AR, Macklin). Methane, carbon monoxide, argon, and oxygen (99.999 vol.%, Qingdao Xin
2
ke yuan) were used as the feedstock gases. All chemicals were used as received without any further purification.
Deionized water was used throughout the research.
Catalyst preparation. RhFe/ZSM-5 was synthesized using a previously reported seeded growth template-free
method with slight modifications. This process included two steps: the synthesis of ZSM-5 seeds and the
synthesis of RhFe/ZSM-5.
In the first step, 15 g of colloidal silica was dissolved in 7 mL of a NaOH solution (1 M) under stirring at
373 K for 1 h. A separate solution was prepared by dissolving 0.45 g of sodium aluminate in 7 mL of a NaOH
solution (1 M). The two solutions were mixed to obtain a synthetic aluminosilicate gel with a molar composition
of 4 Na O:1 Al O :36 SiO :460 H O. The gel was stirred vigorously at 373 K for 2 h and transferred to a stainless-
2 2 3 2 2
steel autoclave for crystallization at 453 K for 48 h. The obtained crystals were collected by filtration, washed
with deionized water, and dried at 373 K to produce the ZSM-5 seeds.
In the second step, a synthetic aluminosilicate gel was prepared using the same method as in the first step.
Subsequently, 1.8 mL of Rh(NO
3
)
3
solution (5% in H
2
O), containing 0.12 g of ferrous chloride (FeCl
2
) and 0.06
g of seeds was added sequentially to the gel. After stirring for 30 min, the mixture was transferred to an autoclave
for crystallization at 453 K for 48 h. The obtained crystals were collected by filtration, washed with deionized
water, and dried at 353 K to obtain the as-synthesized RhFe/ZSM-5. Finally, RhFe/ZSM-5 was converted to its
H-form via ion exchange with ammonium chloride (0.5 M) for 12 h, followed by calcination in air at 823 K for
6 h. The different metal loading in ZSM-5 could be tuned by varying the types of metal precursors while keeping
the other conditions unchanged.
Characterizations. X-ray diffraction (XRD) was performed by the diffractometer (X’Pert PRO MPD,
PANalytical, Netherlands) with Cu Kα radiation (40 kV, 100 mA, λ = 0.154 nm). The scanning electron
microscope (SEM, JSM-7500F, Japan) was utilized to observe material morphology. HRTEM and EDS-mapping
images were captured on the Tecni G30 instrument (FEI, USA). The morphology of the samples was further
observed by aberration corrected high-angle annular dark field scanning transmission electron microscope (AC-
HAADF-STEM, Themis Z, Thermo Scientific, USA). The iDPC-STEM experiments were performed at 300 kV
on a FEI Themis Z microscope equipped with two aberration correctors. The content of metal elements was
determined by the inductively coupled plasma atomic emission spectroscopy (ICP-AES, Agilent 730, USA). The
information on the electronic state of the material surface was collected via the X-ray photoelectron spectrometer
(XPS, ESCALAB 250Xi, Thermo Scientific, USA), and all binding energies were calibrated to the C1s peak of
surface-contaminated carbon at 284.8 eV. Raman spectra were measured on the instrument (InVia Reflex,
Renishaw, England). X-ray Absorption Spectroscopy (XAS) data for Fe and Rh K-edge were collected at the
BL13SSW station of the Shanghai Synchrotron Radiation Facility (SSRF) 43, and data were obtained in
fluorescence excitation mode using a Lytle detector. The X-ray Absorption Near Edge Structure (XANES) and
Fourier-transformed Extended X-ray Absorption Fine Structure (EXAFS) data were analyzed using ATHENA
and ARTEMIS software, respectively, and MATLAB software was employed for the analysis of wavelet
transformed EXAFS data. UV-Vis diffuse reflectance spectra were obtained from the spectrometer (UV-2700,
Shimadzu, Japan) furnished with an integrating sphere device. The fluorescent spectra were collected by the
fluorescence spectrophotometer (RF-6000, Shimadzu, Japan). In-situ diffuse reflectance infrared Fourier
transform spectroscopy (DRIFTS) measurements were measured on the instrument (VERTEX70, Bruker,
Germany), the mercury cadmium telluride (MCT) detector was adopted, H
2
O was bubbled into the reaction by
CH , O , and CO, when the temperature was raised and stabilized to 463 K. The background correction was
4 2
stabilized, CH , O , and CO replaced Ar for testing. Electron paramagnetic resonance (EPR) spectroscopy
4 2
measurement was performed on the instrument (Bruker EMXplus, Germany) with 5,5-dimethyl-1-pyrroline-N-
oxide (DMPO) as the radical trap. The samples were dispersed in H O aqueous solution, dissolved CH and CO
2 2 4
to detect •CH and •OH. The gas chromatography (Scion 456C, Tianmei, China) is equipped with a thermal
3
conductivity detector (TCD), two flame ionization detectors (FID), a methanizer, and a headspace autosampler
(DK-5001A, Beijing Zhongxing, China), which were used to quantify gaseous and CH OH products. High-
3
performance liquid chromatography (HPLC, Prominence-i, LC-2030 Plus, Japan) equipped with a Refractive
Index Detector (RID) was used to quantify HCOOH and CH
3
COOH products. Transmission 57Fe Mössbauer
spectra were collected at 4.2 K with a sinusoidal velocity spectrometer using a 57Co(Rh) source. Velocity
calibration was carried out using an α-Fe foil at room temperature. The source and the absorbing samples were
kept at the same temperature during the measurements. The Mössbauer spectra were fitted using the Mosswinn
4.0 program 44. The experiments were performed in a state-of-the-art high-pressure Mössbauer in-situ cell -
developed at Reactor Institute Delft 45.
Oxidative coupling of methane. The methane carbonylation experiment was carried out in a 50 mL high-
pressure reactor (Shi ji shen lang). The catalyst (10 mg) was uniformly dispersed in 20 mL of distilled water and
sonicated for 15 minutes. The reactor was purged with argon gas to replace the air for 3-5 times. Then, methane
oxygen and carbon monoxide were injected at the required pressure. The reaction was carried out in 463 K for
2 h. After the reaction, when the reactor temperature is reduced to room temperature and then cooled to below
283 K with an ice bath, the gas and liquid were collected.
Cyclic experiment. The recycle test followed the same procedure. After each run, the spent catalyst was
separated using filtration, washed with a large amount of H O, and then dried at 353 K in the vacuum oven for
2
the next cycle. The repeated experiments were carried out under the same experimental conditions to verify the
stability of the catalyst.
•OH detection experiment. Terephthalic acid was utilized as the probe for the detection of •OH via the
production of 2- hydroxyterephthalic acid. Typically, 15 mg catalyst was dispersed in 20 mL terephthalic acid
solution (0.5 mM). After 2 h, a certain amount of the reactant was measured on the spectrofluorometer (RF-6000,
Shimadzu, Japan) after filtering. The excitation wavelength was 315 nm, and the peak value at 425 nm was
observed to semi-quantify the concentration of •OH radicals.
The detection of residual H
2
O
2
. After the reaction, 0.1 mol/L H
2
SO
4
and 0.05 mol/L potassium titanium oxalate
were added to the solution, stirred for one minute, and the absorbance was measured by UV.
Mössbauer Spectroscopy in-situ study. Transmission 57Fe Mössbauer spectra were collected at 4.2 K with a
sinusoidal velocity spectrometer using a 57Co(Rh) source. Velocity calibration was carried out using an α-Fe foil
at room temperature. The source and the absorbing samples were kept at the same temperature during the
measurements. The Mössbauer spectra were fitted using the Mosswinn 4.0 program. The experiments were
performed in a state-of-the-art high-pressure Mössbauer in-situ cell developed at Reactor Institute Delft 45.
DFT. The spin polarization DFT calculations were conducted using the QUICKSTEP module of the CP2K-
2024.3 code 46, which utilizes a combination of Gaussian and plane-wave basis sets. For the exchange-correlation
functional, the Perdew-Burke-Ernzerhof (PBE) approach was implemented. Core electrons were modeled with
Goedecker-Teter-Hutter pseudopotentials, while the wave functions of valence electrons were represented using
a DZVP-MOLOPT-SR-GTH basis sets. 500 Ry cutoff and 60 Ry rel_cutoff was applied, during the calculations,
all the atomic positions were fully relaxed until the force is smaller than 4.5 × 10^-4. The structures were
optimized using the Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm, setting the SCF convergence
criterion to 1.0 × 10^-6. To account for van der Waals (vdW) interactions, the DFT-D3(BJ) method, which
includes an empirical damping term, was added to the electronic energy calculations.
The free energy G is calculated by G = E + ZPE − TS, where E are the DFT based energy, ZPE and TS are
the correction of zero point energy and entropy, respectively. Input files and data post-processing were performed
using the Multiwfn program 47,48.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript “Direct oxidative carbonylation of methane to acetic acid via high-valent iron oxo mediated water oxidation” reports on the use of Rh–O–Fe sites in ZSM-5 for the oxidative carbonylation of methane to acetic acid. The authors claim to introduce a novel paradigm for selective methane carbonylation, achieving an acetic acid production rate of 18.2 mmol g_cat-1 h-1 with 92% selectivity. However, there appears to be overlap with recent studies on Fe- and Rh-modified zeolites for methane carbonylation (doi.org/10.1002/cctc.202000168, https://doi.org/10.1016/j.cattod.2025.115558, https://doi.org/10.1016/j.micromeso.2021.111581, https://doi.org/10.1134/S0965544123060075). These paperes achieved comparable production rates and selectivities through radical-based coupling mechanisms. Thus, although the present study demonstrates improved catalytic performance, it does not appear to represent an entirely new advance in the field. Additional points that require clarification or improvement:\n1. The reaction should inevitably be accompanied by CO oxidation to CO2; however, the authors do not report selectivity toward gas-phase products.\n2. Stability tests are presented up to 100 h but only in continuous flow at much lower productivity than in batch. Why is there such a large discrepancy between batch (18.2 mmol g_cat-1 h-1) and flow (0.71 mmol g_cat-1 h-1)? This requires explanation.\n3. Results are reported per gram of catalyst, but it would be also important to compare catalytic activity based on TOF values per metal site.\n4. The mechanistic assignment relies heavily on Mössbauer and EPR spectroscopy, but the interpretation of Fe(IV)=O as responsible for the direct conversion of H2O to OH radicals seems speculative. The role of the zeolite framework and its acid sites is also unclear.\n5. The EXAFS assignments of peaks to Fe–O–Fe and Fe–O–Rh are not fully convincing. For such assignments, reference samples with the proposed bonding environments are essential.\n6. The respective roles of Rh and Fe remain ambiguous: the text suggests Rh(III) activates CH4 while Fe(IV)=O activates H2O, yet Mössbauer data indicate Fe species are not strongly affected by CH4. More clarification is needed.\n7. No direct detection of CH₃ or COOH radicals intermediates is shown by EPR. Radical trapping experiments are necessary to confirm the presence of these species.", "reviewer_2": "Typical metals like Rh and Fe have been widely reported for methane oxidative carbonylation to produce acetic acid in both CH4/CO/O2 and CH4/CO/H2O2 systems. Rh-Fe dual-atom sites on MoS2 and Ir-O-Ir dimeric sites on SBA-15 have achieved high acetic acid selectivity at low temperatures (J. Am. Chem. Soc. 2025, 147, 14530; J. Am. Chem. Soc. 2023, 145, 769). Here, the authors report Rh-O-Fe dual-metal sites that enable CH3COOH synthesis at higher temperatures (463 K) with high yield and selectivity, typically shows low catalytic activity below 423 K. Unlike the conventional CO insertion into M-CH3, they propose a direct carbon-carbon coupling mechanism, offering a more feasible pathway for acetic acid formation, similar pathway has also been reported (Appl. Catal. B 2025, 378, 125632). Although the findings sound good and the manuscript is meticulously prepared, the overall novelty of this study appears constrained by prior publications in this field. Additionally, several substantive concerns preclude my endorsement for publication in Nature Communications in its current form.\n1. The assertion of Rh-O-Fe structure formation relies primarily on indirect evidence from Fe/Rh K-edge FT-EXAFS and 57Fe Mössbauer spectroscopic analyses. However, these techniques cannot provide unambiguous structural confirmation. To strengthen their claim, the authors should incorporate direct visualization through aberration-corrected HAADF-STEM and additional spectroscopic validation via CO-IR measurements.\n2. The authors propose a reaction mechanism for CH3COOH formation involving direct carbon-carbon coupling between ·CH3 and ·COOH radicals, same as previous study (Angew. Chem. Int. Ed. 2024, 63, e202315343; Appl. Catal. B 2025, 378, 125632). However, this claim requires more substantial evidence. First, the manuscript fails to provide direct experimental evidence for ·CH3 radical capture (EPR). Second, while *COOH intermediates are known to form in CO/O2/H2O systems (ACS Catal. 2020, 10, 13993). From Fig. 3C, the formation of H2O2 is clearly observed, indicating that Rh sites can indeed catalyze the reaction of CO, O2, and H2O to produce H2O2 via *COOH intermediates. However, Fig. 5B presents a strikingly distinct O2 activation pathway on Rh sites for methane oxidation. The authors should provide more evidence about this phenomenon.\n3. The coupling of *COOH with *CH3 species to produce acetic acid needs verification. As a proof of concept, if CH3I (provided CH3 radicals) were added as substrate, significantly enhanced acetic acid production would be expected. Furthermore, following the proposed mechanism, using ethane as substrate should yield substantial amounts of CH3CH2COOH.\n4. The authors propose that the in situ formation of high-valent Fe(IV)=O initiates a truncated WGS pathway, wherein H2O is directly dissociated into ·OH. These radicals rapidly react with CO to form ·COOH intermediates, which then couple with ·CH3 to yield CH3COOH. To validate this mechanism, in situ DRIFTS studies on Fe/ZSM-5 are recommended: Upon sequential exposure to O2 and CO, the formation of ·COOH intermediates should be observable.", "reviewer_3": "In this article, Zhang and coauthors presented Rh-Fe site-supported ZSM-5 towards selective CH4 conversion to CH3COOH. The well-designed RhFe/ZSM-5 exhibits quite high CH3COOH production rate 18.2 mmol gcat-1 h-1, with 92% selectivity, which is much competitive among the current promising catalysts. Great efforts have been devoted to the study on the characterization and involved catalytic conversion process by in-situ Mössbauer spectroscopy, in-situ high-field EPR and in-situ FT-IR, etc. In particular, the mechanism of the •COOH and •CH3 to yield CH3COOH under Rh(III)/Fe(IV)=O active site was revealed. Overall, the conclusion is supported by solid evidence from both comprehensive experimental results and theoretical calculation, with adequate novelty for the material designing, synthesis and insights validation. Hence, I would like to recommend this work to be published after minor revision.\n1. This paper presents a sustainable methane conversion strategy. By comparing it with existing approaches, the work highlights the advantages of the proposed catalytic method, thereby strengthening the manuscript's overall contribution.\n2. The notation for Fe(IV)/Fe4+ and Rh(III)/Rh3+ should be consistent throughout the text.\n3. Through extensive comparative experiments, the authors provide a detailed demonstration of the RhFe/ZSM-5 catalyst's effectiveness. Beyond the comparison of different metals, have the authors considered evaluating other molecular sieves, such as MOR, SSZ-13, or SiO2?\n4. The authors evaluated the catalyst's performance under pressurized conditions. However, further studies under ambient conditions would be beneficial, as they facilitate easier scaling and mitigate the safety concerns of handling high-pressure methane-oxygen mixtures.\n5. The authors convincingly demonstrated the existence of an Rh-O-Fe structure in the catalyst through EXAFS and 57Fe Mössbauer spectra. Have they considered how the catalytic performance would be affected if the Rh-O-Fe structure were absent, or if Rh and Fe were spatially separated? Such an investigation could further corroborate the crucial role of the Rh-O-Fe structure in the oxidative coupling of methane, which would be a very interesting demonstration."}, "author_response": "Dear Reviewers,\nMany thanks for the time and effort you have dedicated to reviewing our\nmanuscript. Based on your valuable comments and suggestions, we have carefully\nrevised the manuscript, supplementing relevant characterization data and discussion.\nThe changes made are highlighted in the revised manuscript and SI in yellow. We\ngreatly appreciate your feedback, which has significantly enhanced the quality of this\npaper. A point-by-point description of the revisions and our responses to the reviewers'\ncomments is provided below.\nResponse to reviewers' comments:\nReviewer #1 (Remarks to the Author):\nThe manuscript “Direct oxidative carbonylation of methane to acetic acid via high-\nvalent iron oxo mediated water oxidation” reports on the use of Rh-O-Fe sites in ZSM-\n5 for the oxidative carbonylation of methane to acetic acid. The authors claim to\nintroduce a novel paradigm for selective methane carbonylation, achieving an acetic acid production rate of 18.2 mmol g -1 h-1 with 92% selectivity. However, there appears\ncat\nto be overlap with recent studies on Fe and Rh modified zeolites for methane\ncarbonylation,\nhttps://doi.org/10.1002/cctc.202000168,\nhttps://doi.org/10.1016/j.cattod.2025.115558,\nhttps://doi.org/10.1016/j.micromeso.2021.111581,\nhttps://doi.org/10.1134/S0965544123060075.\nThese paperes achieved comparable production rates and selectivities through\nradical-based coupling mechanisms. Thus, although the present study demonstrates\nimproved catalytic performance, it does not appear to represent an entirely new advance\nin the field. Additional points that require clarification or improvement:\n1. The reaction should inevitably be accompanied by CO oxidation to CO ; however,\n2\nthe authors do not report selectivity toward gas-phase products.\nResponse:\nWe thank the reviewer for raising this important point regarding the selectivity\ntoward gas-phase products, particularly the potential formation of CO via CO\n2\noxidation. We fully agree that a comprehensive analysis of gas phase products is\nessential for evaluating the overall selectivity and atom economy of the reaction. In\nresponse to the reviewer’s comment, we have conducted additional gas phase product\nanalysis under the different reaction conditions, using gas chromatography equipped\nwith both TCD and FID detectors. The results are summarized below and included in\nthe revised manuscript (Fig. S12-13).\nUnder optimized reaction conditions (463 K, 10 mg catalyst, 2 h reaction time, 20\nmL H O, 30 bar CH , 3 bar O , 6 bar CO), the CO production rate reached 40.6\n2 4 2 2\nmmol·g -1·h-1. Gas-phase product analysis indicated that only CO was detected in the\ncat 2\nsystem, with no other carbon-containing gaseous products observed. Similar amount of\nCO generated, regardless of CH presence, suggest that CO is primarily from the\n2 4 2\noxidation of CO to CO under these conditions.\n2\ncatalyst, 3 h, 20 mL H\n4\n, 3 bar O\n2\n, 6 bar CO. (f) The performance of CO\n2\nunder different temperature.\nFig. S13. Catalytic performance of CO . (A) Different H O volume; (B) Different\n2 2\namount of RhFe/ZSM-5 catalyst; (C) Different reaction time; (D) Different O pressure;\n2\n(E) Different CO pressure; (F) Different CH pressure.\n4\n2. Stability tests are presented up to 100 h but only in continuous flow at much lower\nproductivity than in batch. Why is there such a large discrepancy between batch (18.2\nmmol g -1 h-1) and flow (0.71 mmol g -1 h-1)? This requires explanation.\ncat cat\nResponse: We thank the reviewer for raising this important point regarding the apparent\ndiscrepancy in acetic acid productivity between the batch and continuous flow reactors.\nThe significant difference in the reported values primarily stems from fundamental\ndistinctions in the reactor configurations and operating conditions, rather than\nindicating any inherent instability or performance loss of the catalyst in the flow system.\nWe provide a detailed explanation below:\nReactor configuration:\nThe batch reactor operation was conducted in a liquid-phase slurry system with\nhigh-pressure gases (CH , CO, O ) dissolved in water. This configuration often allows\n4 2\nfor high local concentrations of reactants near the catalyst surface within the confined\nreactor volume, potentially leading to higher observed initial reaction rates per mass of\ncatalyst when measured over short periods (2 hours in our case).\nThe continuous flow stability test was performed in a gas-solid fixed-bed reactor,\nwhere the reactants (CH , CO, O , H O vapor) flow continuously over a stationary\n4 2 2\ncatalyst bed. This mode is more representative of potential industrial application and is\nessential for assessing long-term stability. However, the effective concentration of\nreactants contacting the catalyst at any given moment is typically lower than in a\npressurized liquid-phase batch system.\nOperating Conditions and Space Velocity:\nThe batch reactor productivity of 18.2 mmol g -1 h-1 was achieved under\ncat\noptimized, intensive conditions (30 bar CH , 6 bar CO, 3 bar O , 463 K, 10 mg catalyst\n4 2\nin 20 mL H O) designed to maximize the initial rate.\nstable long-term operation. The weight of catalyst used in the flow reactor (0.2 g) was\nsignificantly larger than in the batch test (10 mg), and it was exposed to a continuous\nflow of reactants. The productivity in a flow reactor is inherently tied to the residence\ntime and the available active sites per unit time. The lower reported value (0.71 mmol\ng -1 h-1) reflects the steady-state productivity under these stable flow conditions. It is\ncat\nnot directly comparable to the initial, high-intensity rate measured in the batch\nautoclave.\nPurpose of the stability test:\nThe primary objective of the 100-hour continuous test was to demonstrate the\nrobust stability and absence of deactivation of the RhFe/ZSM-5 catalyst under practical,\nsteady-state conditions. The key finding is the stable activity maintained over 100 hours\n(Fig. 5C), which is a crucial metric for catalyst viability. The fact that the catalyst did\nnot deactivate confirms its structural integrity, as corroborated by post-reaction\ncharacterization.\nInherent differences in productivity calculation:\nBatch reactor productivity calculations can sometimes reflect initial, potentially\nmass-transfer-limited rates in a closed system. Continuous flow productivity represents\na time-averaged, steady-state conversion in an open system, which is often lower on a\nper-gram-catalyst-per-hour basis but is a more reliable indicator for scalable processes.\nIn summary, the higher productivity in the batch reactor reflects an optimized,\ninitial performance under specific intensive conditions in a liquid medium. In contrast,\nthe lower but perfectly stable productivity in the continuous flow reactor demonstrates\nthe catalyst's ability to function without deactivation under realistic, steady-state gas-\nsolid reaction conditions over an extended period. This discrepancy is common in\ncatalysis research when transitioning from batch screening to continuous flow operation\nand does not detract from the key finding of excellent catalyst stability.\n3. Results are reported per gram of catalyst, but it would be also important to compare\ncatalytic activity based on TOF values per metal site.\nResponse:\nWe thank the reviewer for this valuable suggestion. We agree that assessing the\nturnover frequency (TOF) provides a more fundamental metric for comparing the\nintrinsic activity of the catalytic sites. In response, we have calculated the TOF values\nbased on the total moles of metal loaded (Rh, or Rh+Fe) (Fig. S5). This serves as a\nconservative and widely used estimation method, enabling a more direct comparison of\nsite efficiency.\nThe calculation was performed using the acetic acid production rate under standard\nbatch conditions (463 K, 10 mg catalyst, 2 h, 20 mL H O, 30 bar CH , 3 bar O , 6 bar\n2 4 2\nCO) and the total metal content determined by ICP-AES (Table S3).\nFor the RhFe/ZSM-5 catalyst (0.55 wt% Rh, 0.25 wt% Fe), the TOF was\ncalculated based on the sum of total Rh and Fe atoms. The resulting TOF value is ~216\nh-1. For the monometallic Rh/ZSM-5 catalyst (0.58 wt% Rh), the TOF calculated based\non total Rh atoms is ~92 h-1.\nThe TOF of the RhFe/ZSM-5 catalyst is approximately 2.5 times that of the\nRh/ZSM-5 catalyst. This calculation, based on the total metal loading, provides clear\nevidence that the intrinsic activity per metal atom is significantly enhanced in the\nbimetallic system.\nWe have revised in the manuscript: For the RhFe/ZSM-5 catalyst (0.55 wt% Rh,\n0.25 wt% Fe), the TOF calculated based on total Rh+Fe atoms reached ~216 h-1, which\nis ~2.5 times that of monometallic Rh/ZSM-5 (~92 h-1, calculated on total Rh). This\ndemonstrates a clear enhancement in intrinsic activity per metal atom in the bimetallic\nsystem (Fig. S5).\nFig. S5. TOF in Rh/ZSM-5 and RhFe/ZSM-5.\n4. The mechanistic assignment relies heavily on Mössbauer and EPR spectroscopy, but\nthe interpretation of Fe(IV)=O as responsible for the direct conversion of H O to OH\n2\nradicals seems speculative. The role of the zeolite framework and its acid sites is also\nunclear.\nResponse:\n1. Clarification on the Role of Fe(IV)=O in H O Activation\n2\ncombination of in-situ Mössbauer spectroscopy, isotopic labeling, and kinetic studies,\nwhich collectively provide a consistent picture of the reaction pathway.\nIn-situ Mössbauer Spectroscopy (Fig. 4):\nAfter O treatment, we observed the formation of Fe(IV)=O species (isomer shift\n2\n~0.14-0.21 mm/s), which are stable under reaction conditions. Upon introduction of\nH O, the spectral features change in a manner consistent with the consumption of\n2\nbridging oxygen species and the generation of OH-containing intermediates (e.g., HO-\nFe(III)). This transformation is more pronounced in the RhFe/ZSM-5 sample, indicating\nthat Fe(IV)=O is involved in H O dissociation.\n57Fe Mössbauer spectra were collected at 4.2 K with a sinusoidal velocity spectrometer\nusing a 57Co(Rh) source. Velocity calibration was carried out using an α-Fe foil at room\ntemperature. The source and the absorbing samples were kept at the same temperature\nduring the measurements. (C) In-situ High-field EPR of the catalyst during reaction.\nReaction conditions: 463 K, 12 h, test conditions: 15 K, microwave frequency: 240 Hz.\nIsotopic Labeling (Fig. 3E):\nWhen using H 18O + 16O , the produced CH CO18OH contains 18O in the hydroxyl\n2 2 3\ngroup, confirming that the oxygen in the hydroxyl group originates from water, not O .\n2\nfor •OH formation.\nCHCOOH\n600 3\n300\n0\n4 2 2\nCHCOOH\n3\n600\nu.)\n300\na.\ne n\nsit y\n(\n90\n0\nnt\nI 600\nCHCOOH\n300 3 CH 3 CO18OH\n0\n4 2 2\n3\n0\n58 59 60 61 62 63 64 65\nm/z\nFig. 3. (E) GC-MS spectra of the isotope CH COOH produced from CH conversion\n3 4\nwhen using CH +CO+16O +H 18O, CH +CO+18O +H 16O and CH +CO+O +D O as\n4 2 2 4 2 2 4 2 2\nthe reactants.\nKinetic Isotope Effect (KIE) Studies (Fig. 3B):\nThe inverse KIE (K /K ≈0.21) observed for RhFe/ZSM-5 indicates that H O\nH D 2\nactivation is not rate-determining step in the presence of Fe, consistent with a facile\nFe(IV)=O mediated H O cleavage step.\n2\n20 20\n1 ) CH COOH RhFe/ZSM-5 18\n- at 3\ng c 16\n15\nl\no 14\nm\ns(D O)=0.272\nm 2 12\n(\nd 10 10\nl\ne\ni\ny 8\nH\nO 6\nO 5\nC s(H O)=0.058 4\n3 2\nH\n0 0\n0 20 40 60 80 100 120\nTime (min)\nFig. 3. (B) CH COOH production under H O and D O with different reaction\n3 2 2\ntime over RhFe/ZSM-5.\nControl Experiments with H O (Fig. S33-S34):\n2 2\nAdding H O as an alternative •OH source resulted in lower acetic acid yield and\n2 2\nselectivity, indicating that the Fe(IV)=O pathway is more efficient and selective than free\n•OH generated from H O decomposition.\n2 2\nTherefore, the assignment of the Fe(IV)=O role is not speculative but represents the\nmost consistent interpretation of the combined spectroscopic, kinetic, and isotopic data.\nWe have further strengthened this discussion in the revised manuscript.\nFig. S33. The comparison of H O with CO and O . Reaction conditions: 463 K, 10 mg\n2 2 2\ncatalyst, 2 h, 20 mL H O, 30 bar CH , 3 bar O /3 bar Ar as balance gas, 6 bar CO.\n2 4 2\nFig. S34. The PL spectra of 2-hydroxyterephthalic acid using terephthalic acid as the\nprobe molecule under O + CO and H O .\n2 2 2\n2. The role of the zeolite framework and its acid sites.\nWe fully agree with the reviewer's observation that the role of acidic sites in the\nZSM-5 framework was not sufficiently addressed in the original manuscript. To\nsystematically elucidate their mechanism, we have supplemented the study with the\nfollowing control experiments and characterizations, which further reveal the critical\nrole of acidic sites in this reaction.\nFirst, the functions of the ZSM-5 support are multifaceted, primarily manifested\nin the following two aspects:\n1. Confinement effect and stabilization of active sites\nThe microporous structure of ZSM-5 provides spatial confinement, effectively\nstabilizing the RhFe active sites and inhibiting their aggregation. This has been\nconfirmed by the excellent long-term stability of the catalyst and post-reaction\ncharacterization results.\n(a). Fig. S44 Cycle number of ReFe/ZSM-5 with five reactions. Reaction conditions:\n2 4 2\nThe CH oxidative coupling reaction was catalyzed in a continuous regime. Reaction\n4\nconditions: Before the reaction, 0.2 g of RhFe/ZSM-5 catalyst (20-40 mesh) was\nthoroughly mixed with 0.6 g of acid-washed quartz sand (20-40 mesh) to minimize\ntemperature gradients, and the mixture was loaded into a stainless-steel fixed-bed\nreactor. 503 K, H O 0.3 mL/min, CH 20 mL/min, CO 10 mL/min, O 5 mL/min.\n2 4 2\n2. Catalytic promotion by acidic sites\nTo directly investigate the influence of acidic sites on catalytic performance, we\nprepared a series of RhFe/ZSM-5-X catalysts with identical metal loadings but varying\nSiO /Al O ratios (SiO /Al O = 18, 25, 40, 60, 80). NH -TPD and pyridine adsorption\n2 2 3 2 2 3 3\ninfrared spectroscopy (Py-IR) results show that the acidity of the catalysts significantly\ndecreases with increasing SiO /Al O ratios (Fig. S25A-B).\n2 2 3\nFig. S25. (a) NH -TPD with different SiO /Al O ZSM-5 catalysts. (b) Infrared spectra\n3 2 2 3\nof different SiO /Al O catalysts. (c) The performance of different SiO /Al O catalysts.\n2 2 3 2 2 3\nCatalytic performance tests (Fig. S25C) indicate that both the yield and selectivity\nof CH COOH decrease markedly as the support SiO /Al O increases. Among these,\n3 2 2 3\nRhFe/ZSM-5-18 exhibits the best CH COOH performance (18.2 mmol g -1 h-1), while\n3 cat\nRhFe/ZSM-5-80 shows a substantial decline in activity (0.4 mmol g -1 h-1). This\ncat\nclearly demonstrates that zeolite acidity is a key factor in maintaining high catalytic\nperformance.\n3. Further verification of acidic site function: Na⁺ exchange experiment\nTo further differentiate the acidic contributioin, Na+ ion exchange was performed\non RhFe/ZSM-5 (SiO /Al O =18) to obtain RhFe/Na-ZSM-5. Py-IR reveals a\n2 2 3\nsignificant decrease ino both Brønsted and Lewis acid sites, confirming effective\nneutralization of the acidic sites (Fig. S26A).\nFig. S26. (a) Infrared spectra of pyridine for RhFe/ZSM-5 and Na-RhFe/ZSM-5\ncatalysts. (b) The performance of RhFe/ZSM-5 and Na-RhFe/ZSM-5 catalysts. (c) CO-\nTPD spectra.\nCorrespondingly, the CH COOH yield over RhFe/Na-ZSM-5 decreases to 3.8\n3\nmmol g -1 h-1, further proving that acidic sites (particularly exchangeable H+) are\ncat\nindispensable for high catalytic activity Fig. S26B.\nTo investigate the mechanism behind the activity decline, we conducted CO-TPD\nanalysis (Fig. S26C.). RhFe/ZSM-5 exhibits a strong CO desorption peak at 665 K,\nindicating its strong adsorption capacity for CO. In contrast, the desorption peak for\nRhFe/Na-ZSM-5 shifts to 553 K with reduced intensity, demonstrating that Na⁺\nexchange not only weakens acidity but also the CO adsorption strength of catalyst. The\nreduced CO adsorption capacity may directly affect the formation of the •COOH\nintermediate and the subsequent C-C coupling, thereby lowering the efficiency of acetic\nacid synthesis.\nIn summary, the above experimental results consistently indicate that the acidic\nsites of the ZSM-5 support not only provide a necessary protonic chemical environment\nbut also promote the formation of key intermediates by enhancing CO adsorption.\nThese functions constitute the structural foundation for maintaining the high activity\nand selectivity of this catalytic system.\n5. The EXAFS assignments of peaks to Fe-O-Fe and Fe-O-Rh are not fully convincing.\nFor such assignments, reference samples with the proposed bonding environments are\nessential.\nWe thank the reviewer for this critical comment. We fully agree that having well-\ndefined reference samples with Fe-O-Fe and Fe-O-Rh bonding environments would\nprovide the most direct evidence for our assignment. Unfortunately, synthesizing\nmolecular or solid-state compounds with well-characterized, isolated Fe-O-Rh units is\nextremely challenging, and despite our best efforts, we have not succeeded in obtaining\nthe target compound.\nHere, we wish to emphasize that our assignment of the Rh-O-Fe structure is not\nbased solely on EXAFS fitting, but is robustly supported by a convergence of multiple\nindependent characterization techniques and chemical evidence. To further strengthen\nour conclusion, we have supplemented the evidence with CO-IR spectroscopic data:\nXPS and XANES Analysis (Electronic interaction):\nCompared to Rh/ZSM-5, the Rh 3d XPS binding energy in RhFe/ZSM-5 shifts to\na higher value (307.7 to 308.2 eV) (Fig. S23), while the Fe K-edge in XANES shifts to\na lower energy (Fig. S22). This consistent trend provides strong evidence for electron\ntransfer from Rh to Fe, which is a clear signature of a direct electronic interaction and\nthe formation of Rh-O-Fe linkages, rather than separate Rh and Fe oxide domains.\nFig. S23. Electron transfer. (A) Rh 3d XPS spectra of Rh/ZSM-5 and FeRh/ZSM-5.\n(B) Fe 2p XPS spectra of Fe/ZSM-5 and FeRh/ZSM-5, respectively.\nFig. S22. (A) XANES spectra at the Rh K-edge of the Rh/ZSM-5, FeRh/ZSM-5, Rh O\n2 3\nand Rh foil. (B) XANES spectra at the Fe K-edge of the Fe/ZSM-5, FeRh/ZSM-5, FeO,\nFe O and Fe foil.\n2 3\nSpectroscopic verification of electronic structure via CO‑DRIFTS\nTo further elucidate the unique electronic structure of the catalyst, we carried out\nin‑situ CO‑DRIFTS measurements (Fig. S24). The spectra of ZSM‑5 and monometallic\nFe/ZSM‑5 show characteristic peaks of gaseous CO at 2171 cm-1 and 2118 cm-1,\nwhereas Rh/ZSM‑5 exhibits linear CO adsorption bands at 2107 cm-1 and 2040 cm-1.\nNotably, on the RhFe/ZSM‑5 catalyst, these bands undergo a distinct blue shift to\n2104 cm-1 and 2037 cm-1, indicating a decrease in the electron density of Rh sites. This\ndirectly reflects the electronic interaction between Rh and Fe, specifically, electron\ntransfer from Rh to Fe, and is fully consistent with XPS and XANES data. In addition,\nthe appearance of a bridged CO adsorption peak at 1860 cm-1 further confirms the\npresence of Rh nanoparticles, in agreement with HAADF‑STEM observations.\nFig. S24. CO-IR adsorption spectrum.\nMössbauer Spectroscopy (environment of Fe):\nThe 57Fe Mössbauer spectra measured at 4.2 K (Fig. 2E-F, Table S5) provide\ncrucial complementary information. The significant increase in the proportion of\ndimeric Fe(III) species (from 18% in Fe/ZSM-5 to 28% in RhFe/ZSM-5) strongly\nsuggests that the introduction of Rh leads to the formation of new Fe-containing dimeric\nunits.\nFig. 2. Structural characterization. (E-F) 57Fe Mössbauer spectra for the Fe/ZSM-5\nand RhFe/ZSM-5 catalysts.\nTable S5. The Mössbauer fitted parameters of the Fe-zeolite samples, obtained\nat 4.2 K.\nSample/ Treatment IS QS Hyperfine Γ Phase Spectral\n(mm·s-1) (mm·s-1) field (T) (mm·s-1) contribution (%)\nA. 0.53 0.88 - 1.08 Fe(III)-Da 18\n57Fe-ZSM-5 0.50 0.03 41.6* 0.94 Fe(III)-PHSb 79\n1.38 3.37 0.72 Fe(II)c 3\nB. 0.50 1.06 - 1.09 Fe(III)-D 28\nRh57Fe-ZSM-5 0.50 0.03 41.9* 0.82 Fe(III)-PHS 70\n1.32 3.51 0.72 Fe(II) 2\nExperimental uncertainties: Isomer shift: I.S. ± 0.02 mm s-1; Quadrupole splitting: Q.S. ± 0.05\nmm s-1; Line width: Γ ± 0.05 mm s-1; Hyperfine field: ± 0.2 T; Spectral contribution: ± 3%;\n*Average magnetic field; a Dimeric high-spin Fe(III)-Fe(III) complexes; b Isolated (monomeric)\nFe(III) ions (paramagnetic hyperfine splitting); c Isolated Fe2+ ions.\nFurthermore, compared to Fe/ZSM-5, this dimeric component in RhFe/ZSM-5\nexhibits a lower isomer shift and altered quadrupole splitting, indicating that Fe is in a\ndistinct electronic environment. We attribute this to the formation of mixed Fe-O-Rh\ndimers rather than pure Fe-O-Fe dimers. This interpretation is fully consistent with the\nelectron transfer observed by XPS/XANES.\nCatalytic Performance and Kinetic Isotope Effect (Functional evidence):\nThe dramatic catalytic enhancement (5.7-fold yield increase, Table 1) and the\ndistinct change in the rate-determining step (inverse KIE for RhFe/ZSM-5 vs. normal\nKIE for Rh/ZSM-5, Fig. 3A-B) were not observed with a physical mixture of Rh/ZSM-\n5 and Fe/ZSM-5. This provides compelling functional evidence for a synergistic Rh-Fe\nsite that operates fundamentally differently from isolated Rh or Fe sites. The most\nplausible structural origin for this synergy is the formation of Rh-O-Fe pairs.\nTable 1. Catalytic performance of RhFe/ZSM-5 catalysts for the oxidation of CH\n4 2 3\nT Reactants CH3COOH\nProductivity (mmol gcat -1 h-1)\nEntry Catalyst (K) (MPa) Selectivity\nCH4 O2 CO CH3OH HCOOH CH3COOH (%) d\n9a Rh/ZSM-5//Fe/ZSM-5 463 3 0.3 0.6 0.71 1.42 3.64 63\n10 b Rh/ZSM-5-C 463 3 0.3 0.6 0.4 1.15 1.89 55\n11 b Fe/ZSM-5-C 463 3 0.3 0.6 n.d. c n.d. c n.d. c n.d. c\n12 b RhFe/ZSM-5-C 463 3 0.3 0.6 3.8 1.58 5.08 49\nReaction conditions: catalyst (10 mg), water (20 mL), time (2 h), stirring speed 800 rotations per minute (rpm).\nFor entries 3-5 is the total pressure was maintained with N2 or Ar. a Rh/ZSM-5 and Fe/ZSM-5 were physically\nmixed. b ZSM-5-C was obtained from Nankai University Catalyst Co Ltd, which has the same metal loading\ncapacity and SiO2: Al2O3 ratio (18) as RhFe/ZSM-5. c n.d., not detected. d CH3COOH selectivity in liquid\nproducts.\nFig. 3. CH adsorption and activation mechanism. (A) CH COOH production under H O\n4 3 2\nand D O with different reaction time over Rh/ZSM-5. (B) CH COOH production under H O\n2 3 2\nand D O with different reaction time over RhFe/ZSM-5.\nsample. However, we are confident that the collective body of evidence from XPS,\nXANES, Mössbauer spectroscopy, and the unique catalytic behavior presents a very\nstrong and self-consistent case for the existence of Rh-O-Fe linkages.\n6. The respective roles of Rh and Fe remain ambiguous: the text suggests Rh(III) activates CH\n4\nwhile Fe(IV)=O activates H O, yet Mössbauer data indicate Fe species are not strongly affected by\nMössbauer data show that Fe(IV)=O species are not significantly perturbed by CH\n4\nexposure (Fig. 4A-B, Treatment C), which aligns with our proposed decoupled\nactivation mechanism. In this mechanism:\nFe(IV)=O is responsible for water activation, as evidenced by isotope‑labeling\nexperiments (Fig. 3E), which confirm that the hydroxyl oxygen in acetic acid originates\nexclusively from H O rather than O . Moreover, in‑situ Mössbauer spectroscopy shows\n2 2\nthat Fe(IV)=O species remain largely unaffected upon CH introduction, but disappear\n4\nupon H O exposure, further corroborating the role of Fe(IV)=O in water activation.\n4 4\nB, Treatment C), the spectral contributions of Fe(IV) species remained largely unchanged\nof RhFe/ZSM-5 in Mössbauer spectroscopy, but their hyperfine parameters of Fe(IV)\nshifted more significantly than that Fe/ZSM-5, suggesting that the neighboring Rh sites\nplay a crucial role in C-H activation, rather than the Fe(IV)=O species. Upon CH\n4\nintroduction to the RhFe/ZSM system post O /CO treatment, the HF-EPR signal re-\n2\nemerged (Fig. 4C Treatment C), demonstrating that CH can be activated by high-valent\n4\nmetals (Fe(IV) or Rh(III)). Importantly, the Mössbauer spectra revealed no significant\nchange in Fe(IV) species after CH exposure, confirming that the observed signal in the\nparticipated in CH activation.\n4\nCHCOOH\n600 3\n300\n0\nCHCOOH\n3\n600\nu.)\n300\na.\ne n\nsit y\n(\n90\n0\nnt\nI 600\nCHCOOH\n300 3 CH 3 CO18OH\n0\n4 2 2\n0\n58 59 60 61 62 63 64 65\nm/z\nFig. 3. (E) GC-MS spectra of the isotope CH COOH produced from CH conversion\n3 4\nwhen using CH +CO+16O +H 18O, CH +CO+18O +H 16O and CH +CO+O +D O as\n4 2 2 4 2 2 4 2 2\nthe reactants.\nFig. 4. In-situ Mössbauer spectroscopy. (A) Fe/ZSM-5. (B) RhFe/ZSM-5. Transmission\n57Fe Mössbauer spectra were collected at 4.2 K with a sinusoidal velocity spectrometer\nusing a 57Co(Rh) source. Velocity calibration was carried out using an α-Fe foil at room\ntemperature. The source and the absorbing samples were kept at the same temperature\nduring the measurements. (C) In-situ High-field EPR of the catalyst during reaction.\nReaction conditions: 463 K, 12 h, test conditions: 15 K, microwave frequency: 240 Hz.\nTable S6. The Mössbauer fitted parameters of the Fe/ZSM-5 sample, obtained\nat 4.2 K.\nSample/ Treatment IS QS Hyperfine Γ Phase Spectral\n(mm·s-1) (mm·s-1) field (T) (mm·s-1) contribution\n(%)\nA. 0.21 2.26 - 0.93 Fe(IV)=O (I) cyan 26\n0.2% 57Fe-ZSM-5 0.14 2.28 48.1 0.68 Fe(IV)=O (II) magenta 10\nO2/Ar 0.31 1.44 - 1.15 Fe(III)-Da red 30\n1.36 3.18 - 0.72 Fe(II)c green 4\nB. 0.18 2.13 - 0.88 Fe(IV)=O (I) 19\n0.2% 57Fe-ZSM-5 0.14 2.06 48.9 0.74 Fe(IV)=O (II) 11\nO2/CO/Ar 0.36 1.52 - 1.32 Fe(III)-D 21\n1.13 2.85 - 1.09 Fe(II) 15\nC. 0.19 2.06 - 0.98 Fe(IV)=O (I) 19\n0.2% 57Fe-ZSM-5 0.13 2.08 49.1 0.73 Fe(IV)=O (II) 12\nO2/CO/CH4 0.35 1.32 - 1.14 Fe(III)-D 31\n1.29 2.92 - 0.84 Fe(II) 7\nD. 0.37 1.39 - 0.90 Fe(III)-D 38\n0.2% 57Fe-ZSM-5 0.35 0.03 42.1* 0.79 Fe(III)-PHS 46\nO2/CO/Ar/H2O 1.29 2.70 - 0.98 Fe(II) 16\nE. 0.37 1.30 - 0.93 Fe(III)-D 41\n0.2% 57Fe-ZSM-5 0.36 0.01 43.4* 0.74 Fe(III)-PHS 47\nO2/CO/CH4/H2O 1.35 2.70 - 0.96 Fe(II) 12\nExperimental uncertainties: Isomer shift: I.S. ± 0.02 mm s-1; Quadrupole splitting: Q.S.\n± 0.05 mm s-1; Line width: Γ ± 0.05 mm s-1; Hyperfine field: ± 0.2 T; Spectral\ncontribution: ±3%; *Average magnetic field; aDimeric high-spin Fe(III)-Fe(III) or Fe(III)-\nFe(IV) complexes; bIsolated (monomeric) Fe(III) ions (paramagnetic hyperfine splitting);\ncIsolated Fe(II) ions.\nTable S7. The Mössbauer fitted parameters of the Fe(Rh)/ZSM-5 sample,\nobtained at 4.2 K.\nSample/ Treatment IS QS Hyperfine Γ Phase Spectral\n(mm·s-1) (mm·s-1) field (T) (mm·s-1) contribution\n(%)\nA. Rh57Fe-ZSM-5 0.21 2.27 - 0.82 Fe(IV)=O (I) 16\nO2/Ar 0.16 2.21 48.3 0.65 Fe(IV)=O (II) 10\n0.35 0.03 36.4* 0.75 Fe(III)-PHSb 29\n1.36 3.23 - 0.74 Fe(II)c 2\nB. 0.6% Rh 0.22 2.24 - 0.82 Fe(IV)=O (I) 18\n0.2% 57Fe-ZSM-5 0.18 2.22 48.3 0.55 Fe(IV)=O (II) 9\nO2/CO/Ar 0.32 1.61 - 1.26 Fe(III)-D 31\n1.26 3.09 - 0.99 Fe(II) 7\nC. Rh57Fe-ZSM-5 0.17 1.88 - 1.20 Fe(IV)=O (I) 19\nO2/CO/CH4 0.17 1.98 49.2 0.70 Fe(IV)=O (II) 8\n0.38 0.04 38.6* 0.75 Fe(III)-PHS 32\n1.32 2.81 - 0.99 Fe(II) 12\nD. Rh57Fe-ZSM-5 0.34 1.52 - 1.32 Fe(III)-D 27\nO2/CO/Ar/H2O 0.33 0.01 40.9* 0.97 Fe(III)-PHS 70\nE. Rh57Fe-ZSM-5 0.32 1.54 - 1.12 Fe(III)-D 26\nO2/CO/CH4/H2O 0.36 0.01 41.5* 1.09 Fe(III)-PHS 57\nExperimental uncertainties: Isomer shift: I.S. ± 0.02 mm s-1; Quadrupole splitting: Q.S.\n± 0.05 mm s-1; Line width: Γ ± 0.05 mm s-1; Hyperfine field: ± 0.2 T; Spectral\ncontribution: ±3%; *Average magnetic field; aDimeric high-spin Fe(III)–Fe(III) or Fe(III)–\nFe(IV) complexes; bIsolated (monomeric) Fe(III) ions (paramagnetic hyperfine splitting);\ncIsolated Fe(II) ions.\n7. No direct detection of CH or COOH radicals intermediates is shown by EPR.\nWe thank the reviewer for the valuable suggestion. To directly detect the radical\nintermediates during the reaction, we have conducted in-situ spin-trapping experiments\nusing 5,5-dimethyl-1-pyrroline N-oxide (DMPO) under standard reaction conditions\n(CH /CO/O /H O). The experiment successfully captured radical signals associated\n4 2 2\nwith •OH and •CH , further confirming that the methyl radical is the key intermediate\n3\nin methane activation within this system (Fig. S39).\nFig. S39. In-situ EPR spectra under CH +CO+O +H O.\n4 2 2\nAlthough •COOH signals were not directly observed in the in-situ EPR, we\nperformed supplementary verification of this intermediate using in-situ infrared\nspectroscopy. According to the proposed mechanism, if •COOH is formed via the\ncoupling of •OH and CO on Fe sites, the corresponding intermediate signal should be\ndetectable on the Fe/ZSM-5 catalyst. To this end, we conducted in-situ infrared tests on\nFe/ZSM-5 under an O /CO/H O atmosphere and successfully observed characteristic\n2 2\nvibrational peaks at 1576 cm-1 and 1352 cm-1, which are assigned to the •COOH\nintermediate. This result further confirms that •COOH is a key reaction intermediate in\nthe acetic acid synthesis pathway.\nThe relevant content has been added to the manuscript in Supplementary Figure\nS39-40.\nFig. S40. In-situ diffuse reflectance infrared fourier transform spectra collected\nat the H O +O and O /H O/CO at 463 K over Fe/ZSM-5.\n2 2 2 2\nReviewer #2 (Remarks to the Author):\nTypical metals like Rh and Fe have been widely reported for methane oxidative\ncarbonylation to produce acetic acid in both CH /CO/O and CH /CO/H O systems.\n4 2 4 2 2\nRh-Fe dual-atom sites on MoS and Ir-O-Ir dimeric sites on SBA-15 have achieved\n2\nhigh acetic acid selectivity at low temperatures (J. Am. Chem. Soc. 2025, 147, 14530;\nJ. Am. Chem. Soc. 2023, 145, 769). Here, the authors report Rh-O-Fe dual-metal sites\nthat enable CH COOH synthesis at higher temperatures (463 K) with high yield and\n3\nselectivity, typically shows low catalytic activity below 423 K. Unlike the conventional\nCO insertion into M-CH , they propose a direct carbon-carbon coupling mechanism,\n3\noffering a more feasible pathway for acetic acid formation, similar pathway has also\nbeen reported (Appl. Catal. B 2025, 378, 125632). Although the findings sound good\nand the manuscript is meticulously prepared, the overall novelty of this study appears\nconstrained by prior publications in this field. Additionally, several substantive\nconcerns preclude my endorsement for publication in Nature Communications in its\ncurrent form.\n1. The assertion of Rh-O-Fe structure formation relies primarily on indirect evidence\nfrom Fe/Rh K-edge FT-EXAFS and 57Fe Mössbauer spectroscopic analyses. However,\nthese techniques cannot provide unambiguous structural confirmation. To strengthen\ntheir claim, the authors should incorporate direct visualization through aberration-\ncorrected HAADF-STEM and additional spectroscopic validation via CO-IR\nmeasurements.\nWe sincerely appreciate the reviewer's critical suggestions, which are essential for\nstrengthening the evidence regarding the Rh-O-Fe structure. We agree that direct visual\nobservation and supplementary spectroscopic verification would significantly enhance\nthe persuasiveness of the conclusions. Following these recommendations, we have\nconducted the relevant experiments and present new experimental results below, which\ncollectively provide robust multi‑faceted evidence supporting the existence of Rh-O-\nFe linkages.\n1. Spectroscopic verification of electronic structure via CO‑DRIFTS\nTo further elucidate the unique electronic structure of the catalyst, we carried out\nin‑situ CO‑DRIFTS measurements (Fig. S24). The spectra of ZSM‑5 and monometallic\nFe/ZSM‑5 show characteristic peaks of gaseous CO at 2071 cm-1 and 2118 cm-1,\nwhereas Rh/ZSM‑5 exhibits linear CO adsorption bands at 2107 cm-1 and 2040 cm-1.\nNotably, on the RhFe/ZSM‑5 catalyst, these bands undergo a distinct blue shift to\n2104 cm-1 and 2037 cm-1, indicating a decrease in the electron density of Rh sites. This\ndirectly reflects the electronic interaction between Rh and Fe, specifically, electron\ntransfer from Rh to Fe, and is fully consistent with XPS and XANES data. In addition,\nthe appearance of a bridged CO adsorption peak at 1860 cm-1 further confirms the\npresence of Rh nanoparticles, in agreement with HAADF‑STEM observations.\nFig. S24. CO-IR adsorption spectrum.\n2. Direct Structural Observation via Aberration‑Corrected HAADF‑STEM\nWe performed detailed AC‑HAADF‑STEM analysis. High‑resolution images\n(Fig. S17) clearly show the microporous structure of ZSM‑5, dispersed nanoparticles,\nand single atoms distributed adjacently in multiple regions. By combining the results\nfrom CO-IR and EXAFS spectra (Fig. S24, Fig. 2C-D), the existence of Rh\nnanoparticles and Fe single atoms has been demonstrated. These results directly reveal\nthe spatial proximity between Rh and Fe, corroborating Mössbauer spectroscopy data,\nand providing visual morphological evidence for the Rh-O-Fe coordination structure\n(Fig. 2E-F).\nFig. S17. AC‑HAADF‑STEM of RhFe/ZSM-5.\nIn summary, AC‑HAADF‑STEM imaging provides direct spatial evidence of Rh-\nO-Fe pairing at the atomic scale, while CO‑DRIFTS data reveal the consequent\nmodification of the electronic structure of Rh sites at the spectroscopic level. These new\nexperimental results, together with the previously presented EXAFS and Mössbauer\ndata, form a coherent and self‑consistent body of evidence that clearly confirms the\nformation of the Rh-O-Fe structure in our catalytic system.\nWe have integrated these new findings and related discussions into the revised\nmanuscript. Once again, we sincerely thank the reviewer for their valuable suggestions,\nwhich have significantly enhanced the reliability and completeness of the key structural\narguments in this work.\n2. The authors propose a reaction mechanism for CH COOH formation involving\n3\ndirect carbon-carbon coupling between ·CH and ·COOH radicals, same as previous\n3\nstudy (Angew. Chem. Int. Ed. 2024, 63, e202315343; Appl. Catal. B 2025, 378,\n125632). However, this claim requires more substantial evidence. First, the manuscript\nfails to provide direct experimental evidence for ·CH radical capture (EPR). Second,\n3\nwhile *COOH intermediates are known to form in CO/O /H O systems (ACS Catal.\n2 2\n2020, 10, 13993). From Fig. 3C, the formation of H O is clearly observed, indicating\n2 2\nthat Rh sites can indeed catalyze the reaction of CO, O , and H O to produce H O via\n2 2 2 2\n*COOH intermediates. However, Fig. 5B presents a strikingly distinct O activation\n2\npathway on Rh sites for methane oxidation. The authors should provide more evidence\nabout this phenomenon.\nWe sincerely thank the reviewer for these insightful and constructive comments,\nwhich have enabled us to significantly strengthen the evidence for our proposed\nmechanism and clarify its novelty. We have conducted additional experiments to\naddress these issues.\n1. Direct experimental evidence for ·CH radicals\nproposed mechanism. Following the reviewer's suggestion, we performed electron\nparamagnetic resonance experiments using the spin trap DMPO. When the RhFe/ZSM-\n5 catalyst in a DMPO-containing suspension was exposed to reaction atmosphere, we\nobserved a characteristic EPR signal (Fig. S39). This signal matches the data reported\nin the literature for the DMPO-·CH and DMPO-OH adduct. This provides direct and\n3\nunambiguous evidence for the generation of ·CH radicals from CH activation on our\n3 4\ncatalyst.\nFig. S39. In-situ EPR spectra under CH +CO+O +H O.\n4 2 2\n2. Elucidating the unique O activation pathway and addressing the H O\n2 2 2\nobservation.\nThe reviewer correctly notes that H O formation is observed on monometallic\n2 2\nRh/ZSM-5 (Fig. 3C), indicating H O as the intermadiate product pathway. However,\n2 2\nwe wish to emphasize that our proposed unique pathway (Fig. 5B) relies precisely on\nthe synergistic effect of the Rh-O-Fe dual sites, rather than isolated Rh sites. The key\nevidence is as follows:\nCrucial difference in the dual-site system: in the bimetallic RhFe/ZSM-5 catalyst\n(Fig. 3D), we did not detect H O . This stark contrast with monometallic Rh/ZSM-5\n2 2\n(Fig. 3C) indicates a fundamental change in the reaction pathway within RhFe/ZSM-5.\nFig. 3. (C-D) Detection of H O using in-situ DRIFTS spectra of Rh/ZSM-5 and FeRh/ZSM-5\n2 2\ncatalysts.\nDominance of the Fe(IV)=O-mediated pathway: we propose that in RhFe/ZSM-5,\nO preferentially oxidizes Fe sites to highly reactive Fe(IV)=O species (confirmed by\n2\ngenerate ·OH, bypassing the multi-step pathway via H O formation.\n2 2\nFig. 4. In-situ Mössbauer spectroscopy. (A) Fe/ZSM-5. (B) RhFe/ZSM-5.\nTransmission 57Fe Mössbauer spectra were collected at 4.2 K with a sinusoidal velocity\nspectrometer using a 57Co(Rh) source. Velocity calibration was carried out using an α-\nFe foil at room temperature. The source and the absorbing samples were kept at the\nsame temperature during the measurements.\nTable S6. The Mössbauer fitted parameters of the Fe/ZSM-5 sample, obtained\nat 4.2 K.\nSample/ Treatment IS QS Hyperfine Γ Phase Spectral\n(mm·s-1) (mm·s-1) field (T) (mm·s-1) contribution\n(%)\nA. 0.21 2.26 - 0.93 Fe(IV)=O (I) cyan 26\n0.2% 57Fe-ZSM-5 0.14 2.28 48.1 0.68 Fe(IV)=O (II) magenta 10\nO2/Ar 0.31 1.44 - 1.15 Fe(III)-Da red 30\n1.36 3.18 - 0.72 Fe(II)c green 4\nB. 0.18 2.13 - 0.88 Fe(IV)=O (I) 19\n0.2% 57Fe-ZSM-5 0.14 2.06 48.9 0.74 Fe(IV)=O (II) 11\nO2/CO/Ar 0.36 1.52 - 1.32 Fe(III)-D 21\n1.13 2.85 - 1.09 Fe(II) 15\nC. 0.19 2.06 - 0.98 Fe(IV)=O (I) 19\n0.2% 57Fe-ZSM-5 0.13 2.08 49.1 0.73 Fe(IV)=O (II) 12\nO2/CO/CH4 0.35 1.32 - 1.14 Fe(III)-D 31\n1.29 2.92 - 0.84 Fe(II) 7\nD. 0.37 1.39 - 0.90 Fe(III)-D 38\n0.2% 57Fe-ZSM-5 0.35 0.03 42.1* 0.79 Fe(III)-PHS 46\nO2/CO/Ar/H2O 1.29 2.70 - 0.98 Fe(II) 16\nE. 0.37 1.30 - 0.93 Fe(III)-D 41\n0.2% 57Fe-ZSM-5 0.36 0.01 43.4* 0.74 Fe(III)-PHS 47\nO2/CO/CH4/H2O 1.35 2.70 - 0.96 Fe(II) 12\nExperimental uncertainties: Isomer shift: I.S. ± 0.02 mm s-1; Quadrupole splitting: Q.S.\n± 0.05 mm s-1; Line width: Γ ± 0.05 mm s-1; Hyperfine field: ± 0.2 T; Spectral\ncontribution: ±3%; *Average magnetic field; aDimeric high-spin Fe(III)-Fe(III) or Fe(III)-\nFe(IV) complexes; bIsolated (monomeric) Fe(III) ions (paramagnetic hyperfine splitting);\ncIsolated Fe(II) ions.\nTable S7. The Mössbauer fitted parameters of the Fe(Rh)/ZSM-5 sample,\nobtained at 4.2 K.\nSample/ Treatment IS QS Hyperfine Γ Phase Spectral\n(mm·s-1) (mm·s-1) field (T) (mm·s-1) contribution\n(%)\nA. Rh57Fe-ZSM-5 0.21 2.27 - 0.82 Fe(IV)=O (I) 16\nO2/Ar 0.16 2.21 48.3 0.65 Fe(IV)=O (II) 10\n0.35 0.03 36.4* 0.75 Fe(III)-PHSb 29\n1.36 3.23 - 0.74 Fe(II)c 2\nB. 0.6% Rh 0.22 2.24 - 0.82 Fe(IV)=O (I) 18\n0.2% 57Fe-ZSM-5 0.18 2.22 48.3 0.55 Fe(IV)=O (II) 9\nO2/CO/Ar 0.32 1.61 - 1.26 Fe(III)-D 31\n1.26 3.09 - 0.99 Fe(II) 7\nC. Rh57Fe-ZSM-5 0.17 1.88 - 1.20 Fe(IV)=O (I) 19\nO2/CO/CH4 0.17 1.98 49.2 0.70 Fe(IV)=O (II) 8\n0.38 0.04 38.6* 0.75 Fe(III)-PHS 32\n1.32 2.81 - 0.99 Fe(II) 12\nD. Rh57Fe-ZSM-5 0.34 1.52 - 1.32 Fe(III)-D 27\nO2/CO/Ar/H2O 0.33 0.01 40.9* 0.97 Fe(III)-PHS 70\nE. Rh57Fe-ZSM-5 0.32 1.54 - 1.12 Fe(III)-D 26\nO2/CO/CH4/H2O 0.36 0.01 41.5* 1.09 Fe(III)-PHS 57\nExperimental uncertainties: Isomer shift: I.S. ± 0.02 mm s-1; Quadrupole splitting: Q.S.\n± 0.05 mm s-1; Line width: Γ ± 0.05 mm s-1; Hyperfine field: ± 0.2 T; Spectral\ncontribution: ±3%; *Average magnetic field; aDimeric high-spin Fe(III)–Fe(III) or Fe(III)–\nFe(IV) complexes; bIsolated (monomeric) Fe(III) ions (paramagnetic hyperfine splitting);\ncIsolated Fe(II) ions.\nThis pathway difference is directly reflected in the catalytic performance: the\nacetic acid production rate of RhFe/ZSM-5 (18.2 mmol g-1 h-1) is much higher than that\nof Rh/ZSM-5 (3.2 mmol g-1 h-1), and it exhibits a unique inverse KIE (Fig. 3B),\nindicating that water activation is no longer the rate-determining step.\nFig. 3. CH adsorption and activation mechanism. (A) CH COOH production under H O\n4 3 2\nand D O with different reaction time over Rh/ZSM-5. (B) CH COOH production under H O\n2 3 2\nand D O with different reaction time over RhFe/ZSM-5.\nless efficient pathway involving H O , the construction of the Rh-O-Fe dual site opens\n2 2\na new, more efficient route where O is used to generate Fe(IV)=O, enabling direct water\n2\nactivation for selective carbonylation. We have revised the discussion in the manuscript\nto articulate this contrast and its underlying reasons more clearly.\nFig. 5. (B) Proposed RhFe/ZSM-5 catalytic cycle.\n3. The coupling of *COOH with *CH species to produce acetic acid needs verification.\n3 3\nsignificantly enhanced acetic acid production would be expected. Furthermore,\nfollowing the proposed mechanism, using ethane as substrate should yield substantial\namounts of CH CH COOH.\n3 2\nFig. S42. The liquid products after adding CH I.\nresponse, we have conducted additional experiments involving the introduction of CH I.\n2 2\naddition of 200 μL CH I resulted in a 2.4-fold increase in CH COOH productivity\n3 3\ncompared to the system without CH I (Fig. S42). Notably, methanol formation was also\n3\nsignificantly enhanced (38.6 mmol g-1 h-1). These results indicate that the externally\nsupplied •CH radicals can not only directly couple with •OH to form CH OH, but also\n3 3\nefficiently couple with •COOH intermediates to produce acetic acid, which is consistent\nwith the proposed reaction pathway.\nFig. S15. 1H NMR of CH CH after reaction.\n3 3\nSynthesis of propionic acid using ethane as the substrate\nExperiments using ethane as the substrate further validate the reaction mechanism\n(Fig. S15). Under conditions where ethane replaced methane (C H /CO/O /H O,\n2 6 2 2\nRhFe/ZSM‑5, 463 K), the formation of both acetic acid and propionic acid was detected\nin the reaction system. This result confirms that ethane can generate acetic acid via\ndirect coupling of •CH CH with •OH, while propionic acid is formed through coupling\n2 3\nof •CH CH with •COOH radicals. These findings further support the proposed\n2 3\npathway in the manuscript, in which methane is converted to acetic acid via coupling\nof •CH with •COOH. Additionally, the detection of C oxygenates indicates that C-C\n3 1\nbond cleavage of ethane may also occur under the applied reaction conditions.\n4. The authors propose that the in situ formation of high-valent Fe(IV)=O initiates a\ntruncated WGS pathway, wherein H O is directly dissociated into ·OH. These radicals\n2\nrapidly react with CO to form ·COOH intermediates, which then couple with ·CH to\n3\nyield CH COOH. To validate this mechanism, in situ DRIFTS studies on Fe/ZSM-5 are\n3\nrecommended: Upon sequential exposure to O and CO, the formation of ·COOH\n2\nintermediates should be observable.\nWe sincerely appreciate the reviewer’s insightful suggestions regarding the in situ\nspectroscopic validation of the proposed reaction pathway. In response to your\nrecommendations, we performed systematic in situ DRIFTS experiments on Fe/ZSM‑5\nto directly capture the formation of the •COOH intermediate under sequential exposure\nto O and CO. The results provide strong support for the proposed mechanism.\n2\n•OH and subsequently facilitates CO hydroxylation to form •COOH, we carried out\nstepwise in situ DRIFTS measurements under controlled conditions (Fig. S40). The\nexperimental procedure was as follows: First, water vapor was introduced into the\nsystem via bubbling in a nitrogen stream, and a stable baseline spectrum was recorded.\nSubsequently, oxygen and carbon monoxide were introduced sequentially while the\nspectral changes were monitored in real time.\nThe experimental results show that no significant spectral changes were observed\nupon the introduction of oxygen. However, after CO was introduced, distinct CO\nadsorption peaks appeared at 2170 cm-1 and 2119 cm-1. More importantly, characteristic\nvibrational peaks corresponding to the •COOH species were simultaneously observed\nat 1576 cm-1 and 1352 cm-1. This result directly confirms the pathway involving the\ncoupling of •OH with CO to form •COOH, further supporting the reaction mechanism\nproposed in this work, in which the •COOH intermediate plays a key role.\nThe relevant spectral data have been updated in the (Fig. S40) and are referenced\nand discussed in the corresponding section of the manuscript.\nFig. S40. In-situ diffuse reflectance infrared fourier transform spectra collected at the\nH O +O and O /H O/CO at 463 K over Fe/ZSM-5.\n2 2 2 2\nReviewer #3 (Remarks to the Author):\nIn this article, Zhang and coauthors presented Rh-Fe site-supported ZSM-5 towards\nselective CH conversion to CH COOH. The well-designed RhFe/ZSM-5 exhibits quite\n4 3\nhigh CH COOH production rate 18.2 mmol g -1 h-1, with 92% selectivity, which is\n3 cat\nmuch competitive among the current promising catalysts. Great efforts have been\ndevoted to the study on the characterization and involved catalytic conversion process\nby in-situ Mössbauer spectroscopy, in-situ high-field EPR and in-situ FT-IR, etc.\nIn particular, the mechanism of the •COOH and •CH to yield CH COOH under\n3 3\nRh(III)/Fe(IV)=O active site was revealed. Overall, the conclusion is supported by solid\nevidence from both comprehensive experimental results and theoretical calculation,\nwith adequate novelty for the material designing, synthesis and insights validation.\nHence, I would like to recommend this work to be published after minor revision.\n1. This paper presents a sustainable methane conversion strategy. By comparing it with\nexisting approaches, the work highlights the advantages of the proposed catalytic\nmethod, thereby strengthening the manuscript's overall contribution.\nResponse:\nThe methane one-step oxidation process for acetic acid synthesis demonstrates\nsignificant advantages in terms of direct feedstock utilization, mild reaction conditions,\nprocess simplicity, and environmental friendliness. Using methane, carbon monoxide,\nand oxygen as raw materials, this process directly produces acetic acid in an aqueous\nphase at 190°C, thereby bypassing multiple energy-intensive steps required in the\nconventional methanol carbonylation route, such as syngas production and methanol\nsynthesis. This not only greatly simplifies the process chain and reduces equipment\ninvestment and energy consumption but also avoids the use of corrosive promoters like\nCH I, minimizing equipment corrosion and environmental pollution. Moreover, the\n3\nmild reaction conditions ensure safer and more controllable operations, while the\naqueous-phase system aligns better with green chemistry principles. Although this\ntechnology is still under development and requires further optimization in terms of\nconversion efficiency, catalyst cost, and process scale-up, it holds clear application\nprospects for the valorization of distributed resources such as natural gas and biogas,\noffering a potentially more atom-economical and sustainable pathway for acetic acid\nproduction.\nBased on the reviewers' suggestions, we have analyzed the advantages of current\nmethane conversion technologies and the limitations of methanol carbonylation for\nacetic acid production. In the revised manuscript, we have supplemented the detailed\ndiscussion as follows: The one-step CH oxidation process for CH COOH production\n4 3\nenables efficient synthesis through direct methane activation under mild conditions,\nbypassing the complex steps required in traditional methanol carbonylation processes,\nsuch as methanol synthesis and the use of halogen promoters, thus demonstrating\nsignificant advantages in feedstock economy and process simplicity. Although this new\npathway still requires further refinement, it has already provided a more atom efficient\nand sustainable alternative for the green production of CH COOH.\n3\n2. The notation for Fe(IV)/Fe4+ and Rh(III)/Rh3+ should be consistent throughout the\ntext.\nResponse: We thank the reviewer for this careful observation regarding the consistency\nof our notation. We sincerely apologize for this oversight. We agree that maintaining\nconsistent notation for oxidation states is crucial for clarity and scientific rigor\nthroughout the manuscript.\nIn response to this comment, we have carefully revised the entire text to adopt a\nconsistent format for representing oxidation states. Specifically, we have standardized\nthe notation to use Roman numerals in parentheses, such as Fe(IV) and Rh(III),\ninstead of the mixed use of Roman numerals and Arabic numbers with plus signs (e.g.,\nFe4+, Rh3+). All other metal oxidation states (e.g., Fe(III), Fe(II)) have been adjusted\nto this format accordingly.\nThis change has been implemented across all relevant sections, including the\nAbstract, Introduction, Results, Discussion, and Conclusion.\nWe believe this revision ensures consistency and improves the readability of the\nmanuscript. We have carefully checked the entire text to confirm that all oxidation state\nnotations now follow this unified style.\nThank you again for helping us improve the quality of our manuscript.\n3. Through extensive comparative experiments, the authors provide a detailed\ndemonstration of the RhFe/ZSM-5 catalyst's effectiveness. Beyond the comparison of\ndifferent metals, have the authors considered evaluating other molecular sieves, such\nas MOR, SSZ-13, or SiO ?\nFig. S7. Catalytic performance over different supports, reaction conditions: 463 K, 10 mg\ncatalyst, 2 h, 20 mL H O, 40 bar CH , 3 bar O , 6 bar CO.\n2 4 2\nWe sincerely thank the reviewer for this insightful comment. Your suggestion to\nevaluate different supports is highly valuable, as it indeed helps to more\ncomprehensively reveal the relationship between catalyst structure and performance.\nWe have compared the performance of the RhFe catalyst supported on various carriers,\nincluding MOR, SSZ-13, and SiO . The detailed catalytic data have been compiled in\n2\nthe supplementary figure (Fig. S7).\nThe results demonstrate that RhFe/ZSM-5 exhibits significantly superior catalytic\nactivity and selectivity compared to the other supports. This phenomenon is primarily\nattributed to the unique structural characteristics of ZSM-5: firstly, its suitable pore\nstructure provides an optimal confined environment for the reaction intermediates,\neffectively promoting the C-C coupling between •CH and •COOH radicals; secondly,\n3\nthe acidic nature of its framework effectively stabilizes the atomically dispersed Rh-O-\nFe active sites while avoiding the initiation of undesirable side reactions.\nThese comparative experiments further confirm the unique advantages of ZSM-5\nas a support in our reaction system and highlight the sophisticated synergistic effect\nbetween our designed Rh-O-Fe active center and the ZSM-5 carrier.\n4. The authors evaluated the catalyst's performance under pressurized conditions.\nHowever, further studies under ambient conditions would be beneficial, as they\nfacilitate easier scaling and mitigate the safety concerns of handling high-pressure\nmethane-oxygen mixtures.\nResponse:\nFig. S4. (A) Catalytic performance at different pressure, reaction conditions: 10 mg\ncatalyst, 2 h, 20 mL H\n4\n/O\n2\n/CO=10/1/2.\nWe thank the reviewer for raising this critical point regarding process safety and\npotential scalability. We fully agree that operation under ambient pressure is highly\ndesirable from both practical and safety perspectives. Following this suggestion, we\nhave conducted additional experiments to evaluate the catalytic performance under\natmospheric pressure.\nAs shown in Fig. S4A, when the reaction was carried out at atmospheric pressure,\nthe RhFe/ZSM-5 catalyst still demonstrated measurable activity, producing acetic acid\nat a rate of 0.24 mmol g -1 h-1. This confirms that the catalyst remains fundamentally\ncat\nactive even without applied pressure, though the productivity is significantly lower than\nthat under optimized pressurized conditions (18.2 mmol g -1 h-1). This difference is\ncat\nprimarily attributed to the drastic reduction in the dissolved concentration of the\ngaseous reactants (CH and CO) in the aqueous phase at ambient pressure, as governed\n4\nby Henry's law. The inherently low concentration of reactants at the catalyst surface\nlimits the reaction rate.\n5. The authors convincingly demonstrated the existence of an Rh-O-Fe structure in the\ncatalyst through EXAFS and 57Fe Mössbauer spectra. Have they considered how the\ncatalytic performance would be affected if the Rh-O-Fe structure were absent, or if Rh\nand Fe were spatially separated? Such an investigation could further corroborate the\ncrucial role of the Rh-O-Fe structure in the oxidative coupling of methane, which would\nbe a very interesting demonstration.\nResponse:\nWe sincerely thank the reviewer for their insightful comments. The suggestion to\nvalidate the crucial role of the Rh-O-Fe structure through comparative experiments is\nhighly constructive and directly addresses the core of our mechanistic investigation.\nTo systematically explore this issue, we specifically designed and prepared a\ncomparative catalyst with spatially separated active sites: Fe/ZSM-5 was first\nsynthesized via the in-situ seed method, followed by the introduction of Rh species\nthrough impregnation and subsequent calcination at 550°C, yielding a catalyst with\nspatially separated Rh/Fe sites (denoted as RhFe/ZSM-I).\nFig. S2B. The performance of different metal in ZSM-5.\nThe physically mixed Rh/ZSM-5+Fe/ZSM-5 system showed an acetic acid\nproduction rate of only 3.6 mmol g -1 h-1, comparable to the activity of monometallic\ncat\nRh/ZSM-5 alone. The impregnated RhFe/ZSM-I catalyst exhibited an improved\nproduction rate of 6.7 mmol g -1 h-1. In contrast, the closely coupled Rh-O-Fe structure\ncat\nconstructed in this work achieved the highest production rate of 18.2 mmol g -1 h-1.\ncat\nThis gradient in activity is highly convincing: when Rh and Fe are spatially\nseparated (physical mixture), the synergistic effect completely disappears; even when\npartial contact is achieved via impregnation (RhFe/ZSM-I), the activity remains limited;\noptimal catalytic performance is only achieved when the two elements form an\natomically close Rh-O-Fe structure, enabling efficient coupling of ·CH and ·COOH\n3\nradicals."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have addressed almost all of my comments. However, it is clear that, beyond novelty, a significant issue is the substantial formation of CO2 during the reaction of CO oxidation, reaching up to 50 mmol g-1 h-1, which is even higher than the amount of acetic acid produced. This point must be addressed in the manuscript before acceptance. The amount of CO2 formed is almost twice that of acetic acid, and the text should therefore be corrected, in particular the statement: \"In addition, the similar amount of CO2 generated…\". The authors should report the selectivity to acetic acid either by explicitly accounting for CO2 formation or by clearly distinguishing between liquid-phase selectivity and overall selectivity including CO2.", "reviewer_2": "Further revision has been made to improve the manuscript and it is suggested for acceptance.", "reviewer_3": "The authors provided detailed responses to the questions raised by the reviewers. The quality of the manuscript has significantly improved. Therefore, I recommend publishing this manuscript in Nature Communications."}, "author_response": "Dear Reviewers,\nWe sincerely thank the reviewer for the insightful and constructive comments.\nWe acknowledge the importance of addressing the formation of CO in our catalytic\n2\nsystem and agree that the text should accurately reflect the product distribution.\nBelow we provide our response and the corresponding revisions made to the\nmanuscript.\nReviewer #1:\nThe authors have addressed almost all of my comments. However, it is clear that,\nbeyond novelty, a significant issue is the substantial formation of CO during the\n2\nreaction of CO oxidation, reaching up to 50 mmol g-1 h-1, which is even higher than\nthe amount of acetic acid produced. This point must be addressed in the manuscript\nbefore acceptance. The amount of CO formed is almost twice that of acetic acid, and\n2\nthe text should therefore be corrected, in particular the statement: \"In addition, the\nsimilar amount of CO generated…\". The authors should report the selectivity to\n2\nacetic acid either by explicitly accounting for CO formation or by clearly\n2\ndistinguishing between liquid phase selectivity and overall selectivity including CO .\nWe thank the reviewer for highlighting this important point. We agree that the\nsubstantial CO formation should be clearly discussed and that the wording regarding\n2\nselectivity should be clarified. We have now revised the manuscript as follows:\nIn addition, comparative experiments performed with CH COOH in the presence\n3\nand absence of CO confirmed that CO is predominantly formed through CO\n2\noxidation (Fig. S11b). Under standard methane oxidation conditions, the rate of CO\n2\nproduction reached 41.6 mmol g -1 h-1, surpassing that from CH COOH. For clarity,\ncat 3\nboth liquid-phase selectivity (>92% toward CH COOH among oxygenated products)\n3\nand total carbon selectivity (including CO , 42.3% for CH COOH) are provided (Fig.\n2 3\nS12-S13). These results indicate that, although the catalyst shows high selectivity to\nacetic acid in the liquid phase, the oxidation of CO to CO represents a major\n2\ncompeting pathway and warrants further optimization in future studies.\nFig. S11. (b) The yields of CO when feed acetic acid into the reaction in the presence\n2\nand absence of CO, reaction condition: 463 K, 10 mg catalyst, 2 h, 20 mL H O, 3 bar"}]

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