CO_2 hydrogenation to acetic acid and methyl acetate via tandem catalysis

Domain: physics
Published: 2025-12-12
Editorial outcome: major_revision

Abstract
Selective hydrogenation of CO2 into high-value C2+ oxygenates, such as acetic acid (AA), methyl acetate (MA), and ethanol, is highly attractive yet remains a significant challenge. This study demonstrates an innovative tandem catalysis system for converting CO2 to AA and MA, employing GaZrOx and hydrophobically modified MOR-8-C1@PC (PTFE+CNTs) catalysts. The upstream GaZrOx drives CO2 hydrogenation to produce an effluent gas with a controlled CO/(DME+CH3OH) ratio of approximately 4, facilitating the subsequent carbonylation reaction over the MOR-based catalyst. For the downstream catalyst, surface silanization of MOR zeolite combined with a hydrophobic promoter effectively prevents water (generated during CO2 hydrogenation) from entering the zeolite channels and greatly enhances the carbonylation activity and catalyst stability. Results show that at a CO2 conversion of 27%, the selectivity towards AA and MA in the organics reaches up to 78%, with no catalyst deactivation observed over 50 hours. This strategy offers an efficient approach for converting CO2 into C2+ oxygenates, with promising implications for sustainable chemical production.

Introduction
The widespread utilization of fossil fuels has resulted in a persistent escalation of carbon dioxide (CO2) emissions, substantially contributing to the rise of global temperatures over the last century1,2. Extensive efforts have been devoted to developing CO2 capture and utilization (CCU) technologies. In this context, C1 chemistry, focusing on the conversion of CO2 into valuable chemicals, has attracted significant interest, as it enables the valorization of CO2 while simultaneously mitigating CO2 emissions3,4.

Although significant progress has been made in the conversion of CO2 to C2+ products, the selective hydrogenation of CO2 to C2+ oxygenates remains a major challenge. The inherent inertness of CO2 and the high energy barrier for C–C coupling make the synthesis of C2+ oxygenates particularly difficult5. Most catalytic systems for the direct conversion of CO2 to C2+ oxygenates employ bifunctional catalysts, often containing noble metals such as Rh and Pd, which are known for their strong C–C coupling ability. However, the high cost and limited recyclability of these catalysts hinder their practical application6–8. Alternatively, Fischer–Tropsch catalysts (e.g., Co or Fe) and modified methanol synthesis catalysts (e.g., Cu) have been explored for the conversion of CO2 to C2+ oxygenates but mostly exhibited inferior selectivity toward the desired oxygenate products9–12.

Tandem catalysis, in which different reactions are carried out sequentially in a cascade, offers an alternative strategy to precisely control reaction pathways and intermediate species. Recently, tandem catalytic systems combining metal oxides and zeolites have been successfully applied for the conversion of CO2 into C2+ hydrocarbons, following a methanol-mediated reaction route13. In such a system, methanol is generated on oxides (e.g., In2O3, ZnZrO, and ZnAlOx), followed by C-C coupling over zeolite catalysts (e.g., H-SAPO-34, H-SAPO-18, H-SSZ-13, H-ZSM-5) to produce light olefins14–17, gasoline18, aromatics19–21, and other hydrocarbons22,23. Unlike the Fischer–Tropsch mechanism, the methanol-mediated route helps break the limitations of Anderson–Schulz–Flory (ASF) distribution and enhances the product selectivity24,25. Although tandem catalysis has proven effective for converting CO2 to C2+ hydrocarbons, limited progress has been gained in the efficient and selective transformation of CO2 into C2+ oxygenates (e.g., acetic acid, methyl acetate, ethanol) via this approach.

CO2 can be readily converted to CO through the reverse water-gas shift reaction (RWGS, CO2 + H2 → CO + H2O). Additionally, CO2 can be hydrogenated to CH3OH and dimethyl ether (DME)26,27. Furthermore, methanol/DME can be effectively carbonylated to produce methyl acetate and acetic acid. Through the rational design of tandem catalytic systems that integrate CO2 hydrogenation and carbonylation, the selective conversion of CO2 into C2+ oxygenates, such as methyl acetate and acetic acid, can be envisioned. However, this concept has not been successfully implemented for the selective conversion of CO2 to C2+ oxygenates, likely due to the high oxygen content of CO2 and the associated significant water formation during its hydrogenation. For CH3OH/DME carbonylation, the presence of H2O is known to significantly inhibit zeolite-catalyzed carbonylation activity28. Mechanistic studies indicate that H2O disrupts DME adsorption and subsequent CO insertion processes, thereby suppressing carbonylation activity28,29. These detrimental effects of water are also evident in the triple tandem catalytic system converting syngas to ethanol29. Compared to syngas, CO2 hydrogenation produces substantially more water. The adverse impact of water on zeolite-catalyzed carbonylation becomes more pronounced and must be addressed when designing tandem catalytic systems for CO2 conversion.

Hydrophobic modification of catalysts can mitigate the impact of H2O on catalytic active sites while accelerating surface water diffusion, thereby enhancing reaction performance30–36. Xiao et al. reported a hydrophobic strategy involving physically mixed Fischer–Tropsch synthesis (FTS) catalysts with a hydrophobic promoter to accelerate the rapid diffusion of H2O from the catalyst surface and improve CO2 conversion35. Xu et al. designed a bifunctional catalyst comprising hydrophobic FeNa@Si-c and HZSM-5 zeolite, demonstrating unidirectional diffusion of H2O on the hydrophobic surface, thereby suppressing CO2 production in the water-gas shift (WGS) reaction34. The diffusion and migration behavior of H2O generated in situ on the catalyst surface has been thoroughly studied. However, in complex tandem systems, developing catalyst design strategies to mitigate the adverse effects of H2O remain worthy of further investigation.

Here, we propose a dual-bed tandem catalytic system combining GaZrOx and hydrophobically modified MOR-8-C1@PC for converting CO2 into AA and MA. This system achieves a selectivity of 78% for AA and MA in the organic product distribution at a CO2 conversion rate of 27%, with no catalyst deactivation observed over 50 h. Mechanistic insights from catalytic tests and simulations reveal that the hydrophobic modification of the MOR-8-C1@PC prevents water diffusion in zeolite channels and promotes the quick escape of water into the bulk gas phase, thereby preserving active sites and maintaining catalytic stability.

Results
The tandem catalytic system comprises a bimetallic oxide, GaZrOx, and a hydrophobically modified MOR zeolite. The two catalysts are held in cascade reactors to control the reaction temperatures independently. During the reaction, a mixture of CO2 and H2 atmosphere is introduced into the upstream reactor, where GaZrOx catalyzes both CO2 hydrogenation to CH3OH reaction and the RWGS reaction, generating an intermediate atmosphere of CH3OH/DME and CO37. This intermediate atmosphere further passes through the downstream reactor, where CH3OH/DME carbonylation reaction occurs over MOR zeolite to yield AA and MA (Fig. 1a, Supplementary Fig. 1). From an oxygen balance perspective, an inherent challenge in CO2 hydrogenation reaction is the coproduction of substantial volumes of H2O, due to the high oxygen content of CO2. The excess water poses a significant issue for the proposed tandem catalytic route, as water is known to negatively impact CH3OH/DME carbonylation.

Hydrophobic treatment is thus designed and carried out on MOR zeolite to mitigate the potential adverse effects of water, i.e., Mode 1 to Mode 4 (Fig. 1b). The parent MOR zeolite catalyst is homemade, characterized by high crystallinity and a high percentage of 8-MR acidity. In Mode 1 (M1), the MOR zeolite was ion-exchanged with tetramethylammonium chloride to selectively cover the Brønsted acid sites (BASs) in 12-MR channels (named MOR-8) and directly assembled in the downstream reactor without any hydrophobic treatment. In Mode 2 (M2), surface silanization treatment was further carried out over this reference sample MOR-8 to cover its outer surface with non-polar -CH3, resulting in a MOR zeolite catalyst (named MOR-8-C1) with a hydrophobically modified surface (Supplementary Fig. 2). In Mode 3 (M3), an external hydrophobic promoter composed of polytetrafluoroethylene (PTFE) particles modified with carbon nanotubes (named PC) was introduced and powder-mixed with the reference MOR-8 zeolite obtained in M138,39. Mode 4 (M4) coupled the surface silanization strategy in M2 and the external hydrophobic promoter PC in M3, where the silanized MOR-8-C1 catalyst is powder-mixed with PC in a mass ratio of 1:1, leading to the preparation of a composite catalyst (MOR-8-C1@PC). Under the four modes, the water contact angles of the catalysts were measured as 13°, 101°, 119°, and 129°, respectively (Fig. 1c), certifying the distinct hydrophobic properties of the samples treated in the four different modes.

The powder X-ray diffraction pattern of GaZrOx is displayed in Fig. 1d, which exhibits four reflection peaks (30.3°, 35.3°, 50.6°, 60.2°) that can be well indexed to the tetragonal zirconia phase (t-ZrO2, PDF#50–1089). The interplanar distance of the (011) lattice plane is measured to be 0.295 nm in the high-resolution transmission electron microscopy (HR-TEM) image (Fig. 1f). With the incorporation of Ga, the corresponding t-ZrO2 characteristic diffractions moved to higher angles, which is caused by the smaller radius of Ga3+ ions (0.40 Å) than that of Zr4+ ions (0.82 Å) (Supplementary Fig. 3). The absence of the characteristic peaks of Ga2O3 in the XRD pattern and the uniform distribution of Ga, Zr, and O elements as revealed by TEM-EDS mapping prove that Ga element is uniformly dispersed in the ZrO2 matrix. The X-ray photoelectron spectra (XPS) of GaZrOx, ZrO2, and Ga2O3 are displayed in Fig. 1g and Supplementary Figs. 4 and 5. The binding energy of Ga-2p in GaZrOx is 1117.9 eV, which is lower than the binding energy of individual Ga2O3 (1118.2)40. Meanwhile, the incorporation of Ga causes the Zr 3d signals (181.9, 184.3 eV) of ZrO2 to shift slightly towards the high binding energies. The interaction between Ga and Zr leads to changes in electronic properties, certifying that Ga is successfully added to the ZrO2 lattice to form a GaZrOx solid solution. In the XPS spectrum of O 1s, the characteristic peaks at 530.0, 531.5, and 532.8 eV are attributed to lattice oxygen, oxygen vacancies, and surface-adsorbed oxygen, respectively37,41. With the addition of Ga, the content of oxygen vacancies (Ovacancy/Ovacancy+Olattice) in GaZrOx increases to 28.5%17. The formation of abundant oxygen vacancies can facilitate the chemisorption of CO2 on the catalyst surface and improve the conversion of CO2.

The physicochemical properties of the parent MOR zeolite are given in Supplementary Fig. 6, showing that it has a Si/Al ratio of 15.8 with high crystallinity. N2 sorption revealed a substantial reduction in pore volume and BET surface area of the zeolite following TMA+ ion exchange, attributed to the occupation of acidic sites within the 12-MR channels by TMA+ ions (Supplementary Fig. 7). In contrast, further silanization had a minimal impact on the micropore volume and surface area. Importantly, due to the small molecular size of DME (4.1×1.9×1.5 Å) and MA (5.3×1.9×3.1 Å), the modified zeolite exhibited no pore-blocking effects during the reaction. IGA tests using MA (the largest product molecule in this work) as the probe molecule were conducted to determine the diffusion coefficient before and after hydrophobic modification. The diffusion coefficient only decreased slightly from 2.18×10−2 to 1.12×10−2 cm2 s−1 after trimethoxymethylsilane modification, confirming a negligible impact on mass transport, excluding the possible influences of different diffusion properties of reactant/product molecules over catalytic performances (Supplementary Fig. 8). The FT-IR spectra of the samples are displayed in Fig. 1h. The band corresponding to the Brønsted hydroxyls shifted from 3606 cm−1 (parent MOR) to 3588 cm−1 (MOR-8) after TMA+ exchange. Deconvolution analysis reveals that the percentage of 8-MR BASs rises from 52% to 99% for MOR-8 (Supplementary Fig. 9), certifying that the acid sites in the 12-MR channels could be effectively covered by TMA+ exchange42. Meanwhile, new bands ranging from 2800 cm−1 to 3000 cm−1 appear with the anchoring of TMA cations, corresponding to the C-H vibrations in the methyl group (Fig. 1h)32. Besides TMA+ ion exchange, various alternative strategies, including selective dealumination, pyridine modification, and NH4+ modification, were also tried43–45. Among them, TMA-modified MOR-8 exhibited the lowest undesirable hydrocarbon selectivity and hence was opted as the selective acidity modification strategy for this study (Supplementary Fig. 10).

Regarding surface silanization treatment of MOR-8, a variety of alkylalkoxysilanes with varying alkyl groups were explored as the surface silanization agents. FTIR spectra (Supplementary Fig. 11) show the successful grafting of these alkylalkoxysilanes onto the MOR surface. The MOR-8 sample silanized with propyltrimethoxysilane (MOR-8-C3) exhibited the highest water contact angle (133o), suggesting superior hydrophobicity (Supplementary Fig. 12). The hydrophobicity of sample MOR-8-C16 is reduced, possibly due to the higher steric hindrance and consequent lower grafting degree46. The MOR-8 silanized with methyltrimethoxysilane (MOR-8-C1) also exhibits good hydrophobicity, with the water contact angle measured to be 121°. The thermal stability of the silanized MOR samples was investigated by thermogravimetric and temperature-dependent FTIR analysis (Supplementary Fig. 13). Although MOR-8-C3 demonstrates superior hydrophobicity, the C3 alkyl groups are prone to thermal decomposition (Supplementary Fig. 14). In contrast, MOR-8-C1 exhibits better thermal stability, with the grafted methyl groups being stably preserved at 300 °C. Considering its good hydrophobicity and thermal stability, MOR-8-C1 is thus selected for further study. As shown in Fig. 1h, the 3740 cm−1 band ascribed to the surface silica hydroxyls disappears, accompanied by a synchronous intensity increase of the C-H vibrational band in the range of 2800–3000 cm−1, suggesting that methyl groups were successfully grafted and passivated the surface hydroxyls. The successful grafting of the methyl groups is also corroborated by 13C MAS NMR spectrum (Supplementary Fig. 15), with a resonance peak near 0 ppm ascribed to methyl species. The 1H-29Si cross-polarization (CP) NMR spectrum reveals a significant decrease in the signal attributed to Q3 species (Si(OH)(OSi)3) after silanization, indicating the successful coverage of surface silanol groups. Additionally, new signals corresponding to silane-derived Si species emerge at −56 ppm and −66 ppm, further confirming the modification (Supplementary Fig. 16). The signal of Si(OH)Al at 4.2 ppm in the 1H MAS NMR spectrum of MOR-8-C1, as well as the hydroxyl vibration peak at 3588 cm−1 in the FTIR spectrum, remained unchanged, indicating that the silanization modification of the external surface does not affect the 8-MR BASs (Fig. 1h, Supplementary Fig. 17)47.

The introduction of external hydrophobic agents facilitates rapid diffusion of H2O from catalyst surface and enhances the catalytic performance35. As shown in Supplementary Fig. 18, PTFE exhibits good hydrophobicity with a contact angle of 108°. To further enhance its hydrophobic performance, CNTs were incorporated into PTFE, which increased the surface roughness and raised the contact angle from 108° to 132° (Supplementary Figs. 18, 19)38. In addition, the composite material PC displayed good thermal stability below 723 K (Supplementary Fig. 20). The composite MOR-8-C1/PC (1:1 mass ratio) exhibits the typical diffraction peaks of MOR structure in the XRD pattern (Fig. 1d), suggesting the crystal structure is not altered after surface silanization and powder-mixing with PC (Supplementary Fig. 6). SEM images show that MOR-8-C1 and PC were disorderly mixed in proximity, ensuring the smooth escape of H2O via the gaps between them (Fig. 1e). Sample MOR-8-C1 comprises plate-like crystals with a size of ca. 300 nm (Supplementary Fig. 21). However, although MOR-8-C1/PC exhibited excellent hydrophobic properties (CA=129°), it has an affinity for CH3OH (CA=13.5°), indicating that the hydrophobic modification only alters its affinity towards water, without affecting the reactants (Supplementary Fig. 22).

The catalytic results of four different tandem catalysis modes comprising the sequential assembly of GaZrOx and MOR zeolite catalysts (M1–M4) are illustrated in Fig. 2. Without the assembly of MOR zeolite in the downstream reactor, GaZrOx was ready to catalyze the CO2 hydrogenation reaction with a CO2 conversion of 28%. The resultant outlet atmosphere comprised the unreacted CO2, CO, and a portion of CH3OH and DME, suggesting the co-occurrence of CO2 hydrogenation to CH3OH and RWGS reaction over GaZrOx. The ratio of CO versus CH3OH plus DME was calculated to be ca. 4 in the gas effluents (Supplementary Fig. 23). Although other bimetallic oxides such as InZrO and ZnZrOx also demonstrate good CO2 conversion capabilities and suitable CO/(CH3OH+DME) ratios, notably superior stability is observed for GaZrOx (Supplementary Fig. 24, 25). This difference is particularly evident at the reaction temperature of 350 °C: ZnZrOx suffers from instability due to zinc volatilization and phase segregation, while InZrO promotes excessive hydrocarbon formation, leading to carbon deposition and rapid deactivation. In contrast, GaZrOx achieves a more favorable balance between efficient CO2 activation and long-term stability, accounting for its superior overall catalytic performance22,48. GaZrOx is thus selected as the upstream catalyst of the tandem catalysis system. In situ DRIFTS analysis reveals that CO2 is activated at oxygen vacancies in GaZrOx, generating HCOO* and CH3O* intermediates that are hydrogenated to CH3OH/DME, with the concurrent formation of CO and water via RWGS reaction (Supplementary Figs. 26, 28). The upstream effluent subsequently undergoes carbonylation on the modified MOR zeolite in the downstream reactor, where CH3OH/DME undergoes activation at Brønsted acid sites to form methoxy species, which subsequently react with CO to generate acetyl intermediates and ultimately to form acetic acid (AA) and methyl acetate (MA) (Supplementary Figs. 27, 28).

The water generation rate over GaZrOx was stoichiometrically calculated and listed in Supplementary Table 1. Notable amounts of water were quickly generated under all the investigated conditions, which was expected to severely hinder the subsequent carbonylation of CH3OH/DME over MOR catalysts in the downstream reactor. Over the GaZrOx + MOR-8 tandem catalysis system, an AA+MA selectivity of ~40% was achieved, suggesting the feasibility of CO2 conversion to AA+MA via the designed tandem catalysis system (M1). However, it is also noted that the selectivity towards methanol and DME is over 50% in the organic products, suggesting that a high proportion of methanol/DME produced over GaZrOx in the upstream reactor remains unreacted in the downstream reactor. This result is not unexpected, as the presence of water is known to exert harmful effects on the carbonylation activity of MOR zeolite. When using silanized MOR-8-C1 instead of MOR-8 as the DME carbonylation catalyst, the selectivity toward AA+MA was significantly enhanced to 56% (M2), with the selectivity towards methanol and DME decreased to 36%, implying the improved carbonylation capability of the downstream catalysts. The introduction of external hydrophobic promoters was also beneficial for the DME carbonylation activity, with a selectivity towards AA+MA of 49% achieved (M3). When coupling surface silanization and external hydrophobic promoter modification in Mode 4 (M4), the AA+MA selectivity further rises to 78%, while the selectivity towards methanol and DME drops to 12% (M4). It is noted that the hydrocarbon selectivity is merely 7% over this tandem catalysis system. The above results reveal that different modes of hydrophobic treatment can unequivocally enhance DME carbonylation activity. The resultant hydrophobic surface is speculated to have shielded water from entering the MOR channels and hence preserves its carbonylation activity. The AA/MA ratio can be regarded as an indicator of water inhibition degree, as water is known to react with the acetyl intermediate and consequently generate AA during DME carbonylation49. The AA/MA ratio decreased from 2.3 for M1 to merely 1.4 (Supplementary Table 2), aligned with the increasing hydrophobicity of the four modes shown in Fig. 1c. Noted that, when replacing the GaZrOx by ZnZrOx, the AA/MA ratio rose to 9.0, showing that AA became the dominant oxygenate product (Supplementary Table 3). This difference is attributed to the distinct CH3OH/DME ratios in the upstream effluents of the two catalysts (Supplementary Fig. 24), as CH3OH are mainly carbonylated into AA, while DME favors MA production.

To further explore and elucidate the variations in water molecule diffusion behaviors under different hydrophobic modification conditions, a classical molecular dynamics (MD) modeling study was conducted. The effects of surface silanization are demonstrated through a comparative study of two models, i.e. the hydrophilic and hydrophobic MOR models as illustrated in Fig. 3a, b. Both models are constructed based on the periodic structure of the MOR framework, with a 40 Å thick vacuum nano slit created within, resulting in two distinct surfaces: the upper and bottom surfaces. In hydrophilic MOR, both the upper and bottom surfaces are fully covered with hydroxyl groups (-OH). Conversely, in hydrophobic MOR, these surfaces are entirely modified with -Si(CH3)n(OCH3)m groups, effectively mimicking a silanized environment.

Initially, 480 water molecules were placed within the nano slit and subjected to MD relaxation steps, during which diffusion along the z-direction was restricted to establish a stable initial water distribution configuration along the x- and y-directions. Subsequently, the final MD simulations were conducted using this relaxed configuration to investigate the axial movement of water molecules along the z-axis and the chance of water molecules entering the micropore channels of the MOR zeolite. As shown in Supplementary Fig. 29, water molecules diffuse more quickly into the micropore channels of MOR zeolite when it is not silanized. Specifically, during the first 100 ps of the simulation, ~200 water molecules are observed entering the micropore channels via the hydrophilic surfaces, after which they reach equilibrium. In contrast, for the model with silanized surfaces, this number decreases to only 30–40 molecules. The equilibrium water density contour, illustrated in Fig. 3c, d, highlights the distinct difference between the two models. In the hydrophilic model, water molecules are distributed within the micropores of the MOR zeolite as well as on the hydrophilic surface. whereas in the hydrophobic model, they are primarily confined in the nano slit between the upper and bottom surfaces. To quantitatively describe the anisotropic diffusion behaviors of water molecules, the mean square displacement (MSD) and diffusion coefficient (Ds) along different directions were calculated and shown in Supplementary Fig. 30.

After silanization, Ds in the x- and y-directions increased approximately twofold, whereas in the z-direction, Ds decreased dramatically from 4.8×10−8 m2/s to 1.6×10−9 m2/s. The above results confirmed that surface silanization treatment effectively mitigates water diffusion into the micropore channels of MOR zeolite while facilitating fast diffusion into the stacked voids between zeolite crystals. To clarify the influence of external hydrophobic promoter (PC) on water diffusion behaviors, two additional models (Fig. 3f, g) are constructed and investigated in MD simulation. The models featured the co-existence of four regions (Region I, II, III, and IV) to simulate the physical mixture of MOR zeolite and the hydrophobic promoter. Region I represents the zeolite region, and Regions II and IV simulate the intercrystalline voids between zeolite crystals. Region III incorporates silanized quartz to represent the nonporous external hydrophobic promoter (Fig. 3g), whereas the control model shown in Fig. 3f utilizes only MOR zeolite without any additional promoters. A statistical analysis of the water distribution in the four regions at different time intervals was carried out and illustrated in Fig. 3h. The variation trend in Region I indicates that the number of water molecules decreases faster in the model containing the hydrophobic promoter compared to the control model. This suggests that the hydrophobic promoter facilitates quicker water escape from the zeolite region. This conclusion is further supported by the variation trend in Region IV. In the model containing the hydrophobic promoter, water begins to appear in Region IV as early as 150 ps (Fig. 3h and Supplementary Fig. 31d). In contrast, the number of water molecules remains negligible within the 0–900 ps timeframe for the control model (Supplementary Fig. 31a–c). To further highlight the superiority of the non-hydrophobic additives, we complemented a further modeling study by replacing the hydrophobic additives with hydrophobic MOR zeolite (Supplementary Fig. 32). Compared with hydrophobic MOR model, the mean squared displacement (MSD) of hydrophobic additive model nearly doubled along the x- and y-direction (Supplementary Fig. 33a, b), accompanied by an early appearance of water molecules in Region IV (Supplementary Fig. 33c). These results justify our addition of hydrophobic additives, despite the fact that surface silanization already enhances hydrophobicity. The above analysis concludes that both surface silanization and the addition of an external hydrophobic promoter can alter water diffusion behaviors within the catalysts. Surface silanization effectively mitigates the axial diffusion of water into the zeolite channels, while the external hydrophobic promoter facilitates rapid water escape into the bulk gas phase (Fig. 3e). The close synergy of the two approaches leads to the most enhanced hydrophobic effects in M4 approach.

The reduced amount of water inside the zeolite catalyst, combined with the quick water escape, can significantly attenuate the negative impact of water on the DME carbonylation reaction in the downstream reactor. Consequently, this leads to improved overall catalytic performance for catalyst systems comprising hydrophobically treated zeolite catalysts, including M2 and M3, with even more pronounced effects observed in M4, which combines both surface silanization and the external hydrophobic promoter strategies.

In addition to the intrinsic properties of the catalysts, the reaction conditions are also crucial factors for catalytic performance of tandem catalytic system. Figure 4 illustrates the catalytic performance of the GaZrOx + MOR-8-C1@PC system (M4) with the temperature variation of the upstream and downstream reactors. As the reaction temperature of the upstream reactor increases from 573 K to 673 K, a volcano-like variation trend is observed for the AA+MA selectivity and more pronounced for the space-time yield (STY) of AA+MA. It is acknowledged that DME carbonylation activity is positively related to the partial pressure of CO in the reactant atmosphere. The influence of reaction temperature on CO2 hydrogenation over the single upstream GaZrOx catalyst was also carefully studied (Supplementary Fig. S23). With the rise of reaction temperature, the CO2 conversion shows a monotonous increase from 14% to 37%, with the CO selectivity dramatically rising from 48% to 90%, suggesting that RWGS reaction is favored at elevated temperatures. This trend is further corroborated by thermodynamical simulation, showing that higher temperatures favor the conversion of CO2 to CO, thereby increasing the CO/CH3OH ratio in the products (Supplementary Fig. 34)50. At 573 K, the CO/CH3OH ratio is calculated to be around 1 for the outlet product of and STY for this tandem system. The CO/CH3OH ratio rises with the temperature elevation, enhancing the overall MA+AA selectivity and STY of the tandem catalytic system. However, when the temperature increases to 673 K for the upstream reactor, a significant amount of hydrocarbons dominate the outlet product of the upstream reactor, leading to a reduced AA+MA selectivity. The excessively high CO content, together with the reduced AA+MA selectivity, collaboratively lead to a low AA+MA space-time yield for the tandem system at 673 K (Supplementary Fig. 35). This can well explain the volcano-like variation trend of AA+MA selectivity and STY with the temperature change in the upstream reactor.

The temperature-dependent variation trend for the downstream MOR-8-C1@PC catalyst (M4) is also followed. With the rise in downstream catalyst temperature, a monotonous increasing trend is observed for AA+MA selectivity and STY, corresponding to the superior carbonylation activities of the MOR-based catalysts at elevated temperatures (Fig. 4a). However, with the temperature increases to higher than 563 K, the selectivity toward hydrocarbons starts to be non-negligible, suggesting the occurrence of methanol to hydrocarbons (MTH) side reactions over MOR-8-C1@PC (Supplementary Fig. 36). The optimal reaction temperature for GaZrOx is determined to be 593-623 K to achieve an optimal intermediate atmosphere for the downstream carbonylation reaction. In contrast, a relatively lower reaction temperature of ca. 543–573 K is preferred for carbonylation catalysts to achieve a high DME carbonylation activity and to avoid undesirable hydrocarbon byproducts. The above results highlight the importance of temperature matching over this designed tandem catalytic system and justify the choice of this dual-reactor configuration as our tandem catalytic system.

The reaction pressure also affects the catalytic behavior of the tandem system (Fig. 4c). As the pressure increases, the CO2 conversion rises, whereas the selectivity of CO decreases, indicating an improved conversion of CO2 into organic products. The selectivity of AA+MA in organic products is maintained higher than 80%, even though a slight increase of hydrocarbon selectivity is observed. Notably, higher reaction pressure favors MA production over AA, resulting in elevated MA/AA ratios (Fig. 4c), primarily due to the compositional variations of the upstream effluent. To achieve an optimal space-time yield of AA+MA, a high reaction pressure of 5 MPa is preferred and selected for the tandem catalysis system. The space velocity is revealed to be a key factor influencing product distribution (Fig. 4d). At an excessively high space velocity, AA+MA selectivity decreases, due to the presence of a large amount of unreacted CH3OH+DME. Concurrently, the product distribution shifts towards acetic acid, resulting in a higher AA/MA ratio. Based on the above results, the optimal reaction condition is determined as follows: T(upstream reactor)=623 K, T(downstream reactor)=558 K, P=5.0 MPa, GHSV=1200 mL gcat−1 h−1. Under the optimized reaction conditions, a steady catalytic run of over 50 hours is realized, and the selectivity of AA+MA (excluding CO) is kept higher than 75% during the entire run (Fig. 4e).

The above results demonstrate the successful assembly and feasibility of converting CO2 into C2+ oxygenates via the tandem catalysis concept. The primary C2+ oxygenates produced were MA and AA. To extend the product spectrum, CuZnAlOx, a common hydrogenation catalyst, was incorporated to explore the potential for ethanol production in a resulting triplet tandem catalysis system. As presented in Supplementary Fig. 37, after the assembly of CuZnAlOx catalyst, the MA+AA selectivity in the organic products dropped sharply from ca. 80% to merely 5%. Meanwhile, ethanol emerged as an important product, achieving a selectivity of 50.5% at a CO2 conversion rate of 26%. These results highlight that by further adjusting the tandem catalysis system, the product spectrum can be flexibly shifted from AA+MA towards ethanol, an important and highly demanded oxygenate. As the first demonstration of tandem catalytic strategy for converting CO2 into C2+ oxygenates, this work lacks directly comparable systems in the literature. Results are therefore compared with studies targeting similar products (e.g., ethanol) that proceed via distinct reaction mechanisms. As shown in Supplementary Table 4, the tandem catalysis system integrating metal oxides with zeolites enables effective control of product distribution and enhances selectivity. These comparisons emphasize the potential of the tandem catalytic strategy for CO2 valorization.

Methods
Chemicals and materials
Ga(NO3)3·9H2O (AR, 99.9%), Zr(NO3)4·5H2O (AR, 99.5%), Zn(NO3)2·6H2O (AR, 99%), In(NO3)3·xH2O (AR, 99%), tetramethylammonium chloride ((CH3)4NCl, AR), tetraethylammonium chloride ((C2H5)4NCl, AR), methyltrimethoxysilane (C4H12O3Si, 98%), chlorotrimethylsilane (C3H9ClSi, 98%), propyltrimethoxysilane (C6H16O3Si, 98%), trimethoxyphenylsilane ((CH3O)3SiC6H5, 98%), hexadecyltrimethoxysilane ((CH3O)3SiC16H33, 85%), (3-Aminopropyl) trimethoxysilane (C6H17NO3Si, 97%), PTFE (Polytetrafluoroethylene) aqueous dispersion (60 wt% in water), CNTs (carbon nanotube), SiO2 (AR) and PEEK (Polyetheretherketone) were purchased from Aladdin Chemical Reagent Company. NH3·H2O (25 wt%), pyridine (AR), and NH4NO3 (AR, 99%) were obtained from Sinopharm Chemical Reagent Co. Ltd.

Oxide catalyst preparation for the upper bed
ZrO2, Ga2O3, ZnZrOx (Zn/Zr=0.5) and GaZrOx (Ga/Zn=0.5, 2) were prepared by sol-gel method22. Taking GaZrOx (Ga/Zn=0.5) as an example, 4.18 g of Ga(NO3)3·9H2O and 8.58 g of Zr(NO3)4·5H2O were dissolved in 100 mL deionized water at 353 K under stirring for 30 minutes. Subsequently, 18.0 g of glucose was added to the solution, followed by stirring for 6 h. The obtained product was dried at 393 K overnight and then calcined in air at 773 K for 4 h. In2O3 and InZrOx were prepared by coprecipitation method. 4.82 g In(NO3)3·xH2O (In2O3) or 4.82 g In(NO3)3·xH2O and 8.58 g Zr(NO3)4·5H2O (InZrOx) were dissolved in 100 mL deionized water at 353 K, then NH3·xH2O (5 wt% in H2O) was added dropwise until complete precipitation (pH=9–10). After aging for 0.5 h, the suspension was washed three times with deionized water. The filtered sample was dried at 393 K overnight and calcined in air at 773 K for 4 h. CuZnAlOx (Cu/Zn=2/1 in mole, Al2O3: 11 wt%) was purchased from Yanchang Zhongke (Dalian) Energy Technology Co., Ltd.

Catalyst preparation for the lower bed
A hydrothermal synthesis method was carried out for the preparation of Na-MOR zeolites. Sodium hydroxide, sodium aluminate, tetraethylammonium hydroxide solution (TEAOH, 35 wt%), and H2O were first mixed at room temperature and stirred until completely dissolved. Silica sol was then added dropwise under vigorous agitation. Subsequently, commercial MOR zeolite was added to the above mixture as seeds (4 wt% relative to SiO2). The molar compositions of the synthetic gel is 30 SiO2 : 0.8 Al2O3 : 2.4 Na2O : 1.5 (TEA)2O : 384 H2O. After aging for 1 h under stirring, the final gel was transferred into an autoclave (100 mL) and heated under static conditions at 170 °C for 30 h. Afterward, the crystallized product was washed with distilled water, collected by centrifugation, and dried at 120 °C to get the as-synthesized Na-MOR sample.

HMOR was obtained by ion exchange method. Na-MOR was subjected to ion exchange using 1.0 M NH4NO3 aqueous solution at 353 K for 3 h, with the exchange process repeated twice. The resulting sample was washed 3 times with deionized water, dried at 393 K, and calcined in air at 823 K for 4 h to obtain HMOR.

The synthesis of MOR-8 was obtained by targeted removal of the BASs in 12-MR of HMOR and was prepared by three different methods, denoted as MOR-8 (Py+), MOR-8 (DAl), and MOR-8 (TMA+/TEA+). MOR-8 (Py+) was prepared by pre-adsorption of pyridine. HMOR (40–60 mesh) was heated at 673 K for 2 h in a fixed-bed reactor. After cooling to 573 K, pyridine vapor carried by N2 (100 mL/min) was passed through the HMOR. After treatment for 1 h, the sample was flushed with N2 (200 mL/min) for 1 h. MOR-8 (DAl) was obtained by selective dealumination and steam treatment using MOR-8 (Py+) as precursor. MOR-8 (Py+) was treated in a 0.1 mol/L NaNO3 aqueous solution to exchange the protons in 8-MR. The sample was calcined at 823 K to remove pyridine and then hydrothermally treated at 723 K for 4 h to remove the framework aluminum in 12-MR. After that, the sample repeated the same ion exchange step as HMOR to obtain MOR-8 (DAl).

MOR-8 (TMA+/TEA+) was obtained by ion exchange method. HMOR was dissolved in 1.0 mol/L tetramethylammonium chloride/tetraethylammonium chloride solution, stirred at 353 K for 3 h and repeated 3 times. The sample was then washed 3 times with deionized water and dried overnight at 393 K. MOR-8-R samples were obtained by surface modification of MOR-836, where R refer to different types of silanization reagents. As a typical process for the synthesis of MOR-8-C1, 1 g of MOR-8 was dried in vacuum at 383 K overnight and then dispersed in 40 mL of toluene at room temperature. Subsequently, 1.36 g methyltrimethoxysilane was added. The mixture was stirred at room temperature at 500 rpm for 24 h, then washed 3 times with ethanol. After centrifuging, and drying at 393 K overnight, hydrophobic catalyst MOR-8-C1 was obtained. The synthesis methods for different samples were similar except for using different reagents as precursors. C1: methyltrimethoxysilane. C1Cl: chlorotrimethylsilane. C3: propyltrimethoxysilane. C6: trimethoxyphenylsilane. C16: hexadecyltrimethoxysilane. NC3: (3-Aminopropyl) trimethoxysilane. H-MOR-R samples were obtained by surface silanization treatment of HMOR, and the synthesis procedure was consistent with that of MOR-8-R.

PTFE-CNTs was obtained by evaporating CNTs and PTFE in ethanol solution. 0.6 g CNTs and 8.0 g PTFE aqueous dispersion were mixed in 50 mL of ethanol, and the mixture was stirred at 353 K until the ethanol was evaporated to dryness. After drying at 393 K overnight and calcined in air at 573 K for 4 h, powered PTFE-CNTs was obtained (denoted as PC). PTFE was obtained by the same method without adding CNTs in the process. SiO2-C1 was obtained by surface silanization treatment of fumed silica. The obtained sample was washed with ethanol for 3 times, dried at 393 K, and calcined in air at 573 K for 4 h. MOR-8-C1@PC was prepared by powered mixing 1 g of MOR-8-C1 and 1 g of PTFE-CNTs in an agate mortar grinding for 10 minutes, pressing the tablet under the pressure of 20 MPa and granulating to 40–60 mesh.

Catalyst performance evaluation
The reaction unit consists of two fixed-bed reactors, the temperature of each reactor can be controlled independently (Supplementary Fig. 1). The reactor was equipped with a quartz liner with an inner diameter of 10 mm. In a typical reaction setup, 0.5 g of GaZrOx was loaded into the upstream reactor, while 1 g of MOR-8-C1@PC was placed in the downstream reactor. When two beds were used, they were separated by a 3 mm layer of quartz wool. Prior to the reaction, the catalysts were pretreated under a 30 mL/min flow of 10% H2/90% N2 at 673 K (upstream) and 553 K (downstream) for 2 h, followed by cooling to the desired reaction temperature. The feed gas (23.75% CO2, 71.25% H2, 5.0% Ar) of 21 mL/min were then introduced into the reactor, and the system was pressurized to 5 MPa. Data were collected after a stabilization period of at least 2 h. Online product analysis was conducted using an Agilent 7890B gas chromatograph equipped with HP-PLOT/Q capillary column and TDX-1 packed column, connected to FID detector and TCD detector, respectively. The TCD detector was used to analyze CO2, CO, and Ar, while the FID detector quantified oxygenates and hydrocarbons. CO2 conversion and product selectivity were calculated on a molar carbon basis. Organic product selectivity was determined by excluding CO. The specific calculation formulas are as follows:

CO2 conversion was calculated on a carbon atom basis, that is,
ConvCO2 = (CO2inlet - CO2outlet) / CO2inlet × 100% (1)

The CO selectivity was calculated according to
SelCO = COoutlet / (CO2inlet - CO2outlet) × 100% (2)

The selectivity for each organic product X as determined on the basis of the number of C atoms,
SelProductX = PProductXoutlet / ΣnCnHmOx × 100% (3)

The explanations of the symbols used in Eqs. (1)–(3) are as follows:
CO2inlet: the mole of CO2 in the reactor inlet.
CO2outlet: the mole of CO2 in the reactor outlet.
COoutlet: the mole of CO in the reactor outlet.
PProductXoutlet: the mole of C atoms in organic product X.
ΣnCnHmOx: the total C moles in all organic species. n is the number of C atoms in the organic products (including hydrocarbons).

Catalyst characterization
The crystallinity of the catalyst samples was characterized by a PANalytical X’Pert PRO X‐ray powder diffraction (XRD) meter using a CuKα (λ=0.154 nm) radiation source with a scanning angle (2θ) range of 5°–80°. The composition of samples was determined by Philips Magix-601 X-ray fluorescence (XRF). The topological images of samples were obtained by a Hitachi SU8020 scanning electron microscope (SEM). Transmission electron microscopy (TEM) images and corresponding EDS elemental mapping images were obtained on a JEM-2100F electron microscope with an acceleration voltage of 200 kV. Textural properties of the samples were estimated from nitrogen adsorption-desorption isotherms measured by Micromeritics ASAP 2020 instrument. All samples were outgassed at 553 K for 3 h before the measurement. A Bruker Tensor 27 spectrometer equipped with a mercury-cadmium-telluride (MCT) detector was used for the measurement of Fourier transform infrared spectra (FTIR). The samples were pressed self-supported wafers and placed in the in situ quartz cell. Subsequently, the wafers were dehydrated at 553 K in a vacuum for 30 min to remove the physiosorbed water. After cooling to room temperature, the spectra were collected by a MCT detector in the range of 4000–400 cm−1. The thermal stability of the catalysts was analyzed by the thermogravimetric analysis (TGA, TA Q-600 analyzer) with a heating rate of 10 K/min from room temperature to 1200 K under air flow. The X-ray photoelectron spectroscopy (XPS) analyses were conducted utilizing an ESCALAB 250Xi spectrometer with monochromatized Al Kα excitation radiation. Energy calibration was performed by referencing the C 1s photoelectron peak at 284.8 eV to compensate for surface charging effects. The uptakes of MA on the zeolites were carried out by an intelligent gravimetric analyzer (IGA) equipped with a mesh-type sample cell. The sample (20 mg) was added to the chamber and evacuated at 553 K for 4 h. Adsorption mass changes were recorded at 298 K under controlled pressures (0–10 mbar). 1H, 13C and 29Si MAS NMR were performed on a Bruker Avance NEO 500 spectrometer equipped with 11.7 T and 89 mm wide-bore magnet using a 3.2 mm H/F-X-Y triple resonances MAS probe with the corresponding Larmor frequency. Water-droplet contact angles were performed with Belsorp MaX II system. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was performed on a Bruker Tensor Ⅱ spectrometer equipped with a high-pressure reaction cell and an MCT detector. The reaction cell pressure was controlled by a back-pressure valve. Spectra were collected with a resolution of 4 cm⁻¹.

Computational models and methods
To investigate the diffusion of water molecules into channel of MOR zeolite, two MD boxes with dimensions of around 73×82×150 Å3 are created, in which crystalline MOR structures are occupied. A nano slit with thickness of around 40 Å is generated in the center of MOR, and 480 water molecules are randomly inserted into as-generated nano slit. To study the effect of surface properties of MOR on the diffusion behavior of water molecules, the surface of MOR is chemically-bonded by hydrophilic -OH and hydrophobic -CH3 groups for comparison. Moreover, to further investigate the escape of water molecules from MOR, two another MD models are constructed. The two MD models are created with dimensions of around 41×40×292 Å3, in which four regions are divided; The bottom region is occupied by MOR with nano slit that contains water molecules source. The z-directional thickness of MOR is set to be around 30 Å. The nano slit surfaces of MOR are saturated by the hydrophilic -OH groups. The second region is generated as the vacuum space with z-directional thickness of around 27 Å. The third region is composed of MOR with length of 192 Å, in which a nano slit is created as the bottom region. The surface of nano slit-contained MOR is chemically modified by either hydrophilic -OH or hydrophobic -CH3 group to compare the water escape. The top region is set as vacuum region which is generated for the escape region of water molecules.

MD simulations are performed to relax confined water molecules with 1,000,000 timesteps at temperature of 523.15 K under NVT (constant number of particles, constant volume, and constant temperature) ensemble, in which the temperature is controlled by Nose-hoover thermostat. MOR-based systems are optimized to a local configuration with energy and force tolerances of 1.0×10−4 kcal/mol and 1.0×10−4 kcal/mol·Å, respectively. To describe the atomic interactions in the MOR-based systems, the consistent-valence forcefield (CVFF) potential is utilized51. During the MD relaxations, water molecules are not allowed to diffuse in the z-direction. Finally, MD simulations are performed at temperature of 523.15 K under NVT ensemble to investigate the diffusion of water molecules into the channels of MOR, and the escape of water molecules from the inner nano slit to long-length nano slit of MOR. All the MD calculations are implemented using the Large-scale Atomic-Molecular Massively Parallel Simulator (LAMMPS) software package (software version: 19 Nov 2024-MSMPI)52.

MSD describes the average squared displacement of the positions of all particles in the system over time from their initial positions. The MSD of water was defined as Eq. (4) in the following:
MSD(τ) = 1/N ΣNi=1 <|ri(t+τ) - ri(t)|2> (4)
where N was the number of molecules, ri(t) and ri(t+τ) denote the position vectors of particle I at the initial time t and t+τ, respectively. The diffusion coefficient Ds can be calculated by fitting the slope of the MSD curve according to Eq. (5).
Ds = MSD / 2dt (5)
where d was the dimension of materials. For a three-dimensional system, d=3, while for a two-dimensional system, d=2.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The production of oxygenates via CO2 hydrogenation is an important research direction for converting CO2 into high-value-added chemicals. In this process, the effective utilization of oxygen in the products presents superior advantages compared to the production of hydrocarbons such as olefins and aromatics from CO2. In this work, a tandem catalytic strategy was employed to achieve the conversion of CO2 to acetic acid and methyl acetate via CO/(DME + CH3OH) intermediates. Although the composition controllability of the raw materials formed from the upper bed layer needs to be further improved for the tandem catalysis, the preliminary coupling of CO2 hydrogenation and carbonylation reactions was realized through the regulation of reaction conditions and the hydrophobic design of zeolite catalyst surfaces. A CO2 conversion of 27%, the selectivity towards AA and MA in the organics up to 78.8%, with no catalyst deactivation observed over 50 hours has already been achieved. It should be noted that this work had carried out various hydrophobicity modifications and kinetic calculations, which demonstrates a high depth of theoretical understand, although the influence of the change in catalyst hydrophobicity on the reaction efficiency and product formation has been reported for other reactions(such as Fang et al., Science 377, 406–410 (2022). This manuscript is well-organized and great efforts have been devoted to enrich the work, and it cannot be denied that there are some attractive points.However, major revisions are still needed for more in-depth research. It could be accepted after reasonably replying the following comments.\n1.The formation of carbon monoxide from the CO2 hydrogenation process is important for the subsequent carbonylation reaction. How the controllable CO formation could be achieved based on conventional GaZrOx catalysts to meet the requirements of downstream reactions.\n2. No additional results regarding the product distribution of oxygenates were obtained. Whether modulation of oxygenate products can be achieved under the current tandem catalytic system requires further discussion.\n3. The stability test of the catalyst was only performed for 70 h. Please authors provide more compelling stability data.\n4.Compared to the GaZrOx catalyst, both ZnZrOx and InZrOx exhibited significantly reduced catalytic activity and higher initial activity. Further in-depth discussion on this differential behavior is requested.\n5. To enhance the hydrophobicity of the zeolite MOR surface, the integration of M2 and M3 strategies was employed, improving oxygenate selectivity under the M4 approach. Further explanation regarding this synergistic effect is requested. In addition, the effect of the introduction of carbon tubes should be expounded in the content.\n6.For Strategy 4, no significant enhancement in CO2 conversion was observed. Simultaneously, greater yields of target products were obtained, which reflected a strong reaction-pulling effect. Could this indicate that the hydrophobic modification strategy may adversely affect reaction diffusion and suppress feedstock conversion under reaction-pulling conditions?\nFurther discussion on this phenomenon is requested.\n7.Although good tandem conversion performance has been achieved on CO2 conversion and oxygenate selectivity, more comparisons should be provided with relevant reports.\n8. More explaination about the formation processes and regulation mechanisms for different oxygen-containing compounds should be provided.", "reviewer_2": "The manuscript entitled “CO2 Hydrogenation to Acetic Acid and Methyl Acetate via Tandem Catalysis” presents an innovative and well-structured approach to convert CO2 into value-added C2+ oxygenates through a dual-bed tandem catalytic system. The upstream GaZrOx catalyst efficiently converts CO2 to a CO/CH3OH/DME intermediate gas mixture, while the downstream hydrophobically modified MOR zeolite (MOR-8-C1@PC) enables selective carbonylation to methyl acetate and acetic acid. The hydrophobic engineering strategy to mitigate water inhibition is scientifically sound and well-substantiated through molecular dynamics simulations and surface analyses. Overall, the study provides substantial insight into the integration of tandem catalysis and water management for efficient CO2 utilization. However, to further clarify and strengthen the manuscript, the following minor revisions and scientific questions are suggested:\n1.The MD simulations show reduced water ingress into hydrophobic MOR channels, but how does hydrophobicity precisely preserve active sites? Are there experimental data (e.g., in situ IR spectroscopy) confirming reduced water adsorption on BAS in 8-MR pockets? Why does MOR-8-C1 (CA=121°) outperform MOR-8-C3 (CA=133°) in stability despite lower hydrophobicity? Is this solely due to thermal stability, or do methyl groups offer unique steric/electronic effects?\n2.GaZrOx achieves a CO/(DME+CH3OH) ratio of ~4. How was this ratio identified as optimal? What happens if the ratio deviates (e.g., higher CO or DME)? Why does GaZrOx show superior stability over InZrOx/ZnZrOx? Is Ga3+ doping critical for oxygen vacancy stability?\n3.Hydrocarbon selectivity rises at >563 K (Fig. 4a). Is this purely due to temperature-dependent MTH activation?\n4.1H MAS NMR shows unchanged Si(OH)Al peaks post-silanization, but does reaction-induced hydrolysis regenerate surface hydroxyls? Provide 29Si NMR data after catalysis to confirm silane stability.\n5.What is the long-term impact of accumulated water or possible coke formation on the structural and catalytic stability of MOR-8-C1@PC, and how can it be mitigated in industrial settings?\n6.How does the hydrophobic modification affect the adsorption/desorption dynamics of key intermediates (e.g., DME, CH3OH, CO) beyond just water diffusion, and can this influence product distribution or selectivity under different reaction conditions?\n7.Abbreviations: Define all acronyms (e.g., PC, MTH, STY) upon first use.\n8.The phrase “facilitating he subsequent” in the Abstract contains a typo. Revise to “facilitating the subsequent” for grammatical accuracy."}, "author_response": "Response: We sincerely appreciate the reviewer for the positive comments and constructive suggestions for improving this manuscript.\nComments 1: The formation of carbon monoxide from the CO hydrogenation process is important for the subsequent carbonylation reaction. How the controllable CO formation could be achieved based on conventional GaZrO catalysts to meet the requirements of downstream reactions.\nResponse: Thanks very much for your positive comments and your question. In our study, we evaluated the capacity of different bimetallic oxides, among which ZnZrO , GaZrO , and InZrO have been reported to exhibit particularly high CO hydrogenation activity (ACS Catal, 2019, 9, 10253-10259), to prepare CH OH and CO with a controlled composition. Among the above oxide catalysts, GaZrO was selected as the catalyst for the first stage of our tandem system, due to its high activity and long-term stability. Mechanistically, the Zr3+−O −Ga−O structures serve as the primary active sites on GaZrO . H dissociates on Ga–O bonds to form Ga–H species, while CO₂ is captured by oxygen vacancies and subsequently activated by Zr3+ centers to form the HCOO* intermediate. Stepwise hydrogenation of HCOO* leads to CH O* and ultimately methanol. In parallel, the generation of CO originates from the cleavage of HCOO* into CO* and OH*, followed by desorption to release CO and H O (ACS Catal. 2021, 11, 4704−4711, Nat. Catal. 2022, 5, 1038-1050, Appl. Catal. B: Environ. Energy 2022, 305, 121042).\nControllable CO formation over conventional GaZrO catalysts is achieved through modulation of reaction conditions. Temperature serves as a key parameter for regulating product distribution at the upstream reactor outlet. Thermodynamic analysis indicates that higher temperatures favor CO -to-CO conversion via the RWGS reaction, while lower temperatures favor the stabilization of formate intermediates and consequently CH₃OH/DME formation. As shown in Supplementary Fig. 23, at 573 K the CO selectivity is approximately 45%, corresponding to a CO/(CH₃OH + DME) ratio of 0.9. Increasing the temperature to 673 K raises the CO selectivity to 94%, yielding a CO/(CH₃OH + DME of 15.6. For the present work, an intermediate temperature of 623 K was selected to maintain the CO/(CH₃OH + DME) ratio within the optimal range for subsequent carbonylation over zeolite catalysts. These results demonstrate that controllable CO production can be achieved by rational catalyst design and careful adjustment of reaction parameters, thereby ensuring compatibility with downstream carbonylation process.\nComments 2: No additional results regarding the product distribution of oxygenates were obtained. Whether modulation of oxygenate products can be achieved under the current tandem catalytic system requires further discussion.\nResponse: We thank the Reviewer for this valuable suggestion. In our tandem system, methanol and DME produced in the upstream reactor are actually intermediate products, which undergoes further carbonylation in the downstream reactor, ultimately yielding methyl acetate (MA) and acetic acid (AA) as our target products. In fact, as the unreacted DME/methanol from outlet of the downstream carbonylation reactor can be recycled into the carbonylation units in future system design, we are particularly interested in the AA/MA ratio in terms of oxygenate product distribution. Just as the Reviewer concerned, modulation of oxygenate products can indeed be achieved, either via the choice/hydrophobic modification of catalysts or via the modulation of reaction conditions.\nFirstly, the choice of the upstream catalyst can effectively modulate the CH OH/DME ratio of the outlet gas from the upstream reactor. As shown in Supplementary Fig. 24, when using ZnZrO instead of GaZrO as the upstream catalyst, a higher proportion of CH OH/DME can be achieved. The variation of CH OH/DME ratio consequently led to a different AA/MA ratio in the final products for our tandem catalysis system (Supplementary Table 2/Table R1), as CH OH are mainly carbonylated into AA while DME favors MA production.\nThe hydrophobic modulation of the downstream catalysts (M1–M4) can also alter oxygenate distribution. With the increasing hydrophobicity, a lower AA/MA ratio is clearly shown in (Supplementary Table 2). This trend is reasonable as water is known to react with the acetyl intermediate and consequently generate AA during DME carbonylation. The increased hydrophobicity can effectively mitigate the influence of water and consequently a decreased AA/MA ratio.\nReaction conditions, such as reaction pressure and space velocity, are also crucial factors influencing the distribution of oxygenate products. In the revised Fig. 4c and 4d, the respective selectivity of AA and MA are illustrated, clearly showing that the relative selectivity of AA and MA indeed depends on reaction conditions. With the increase of reaction pressure from 1 MPa to 5 MPa, an increased proportion of MA product can be observed. In contrast, an increased space velocity leads to a decreased MA proportion. The primary reason lies in the varying ratios of CO, DME, and CH₃OH generated over GaZrO under different reaction conditions, which subsequently determine the distribution of AA and MA after carbonylation.\nThe above results demonstrate that modulation of oxygenate products, specifically the relative distributions of AA and MA, can indeed be achieved within the current tandem catalytic system\nBeyond MA and AA, we also demonstrated ethanol production could be achieved by the further incorporation of CuZnAlOₓ as the hydrogenation catalyst to form a triple tandem catalysis system (Supplementary Fig. 37), indicating the system’s potential to deliver a broader range of oxygenates through adjustments in catalyst composition and bed configuration.\nThe corresponding discussions and modifications have been highlighted in the revised manuscript: Line 12-18, Page9; Line 15-17, 18-21, Page15. Table R1 has been added to the Supporting Information as Supplementary Table 3.\nTable R1 Catalytic performance and product distribution of the GaZrO / ZnZrO + MOR-8-C1@PCa\nCO CO Organic Product Selectivity b\nconv. sel. HC CH OH DME AA MA AA/MA\nx\n30.5 70.5 9.5 13.5 1.8 70.2 5 9.0\nC1@PC\nGaZrO + MOR-8-\nx\n27.5 68.0 7 5 9.4 45.5 32 1.4\nC1@PC\na: Reaction conditions: consistent with the Supplementary Table 2, the data were collected after 10 h of reaction.\nComments 3: The stability test of the catalyst was only performed for 70 h. Please authors provide more compelling stability data.\nResponse: We thank the Reviewer for raising this crucial point. Although the initial 50 h stability test at 623 K demonstrated the peak MA+AA selectivity, this elevated temperature inevitably compromises durability over extended periods by inducing the gradual decomposition of the TMA modifiers.\nTo address this concern, we carried out an additional long-term stability experiment under milder conditions. While keeping the GaZrO catalyst unchanged, we lowered the temperature of the downstream carbonylation bed to 548 K. This adjustment, as shown in Figure R1, significantly enhanced the system's durability, demonstrating stable performance for over 100 hours without observable deactivation. This enhanced durability comes with only a slight reduction in maximum MA+AA selectivity.\nFigure R1 Long-term stability test of tandem system. Conditions: H /CO /Ar = 71.25/23.75/5, T 2 2 (upper bed) = 623 K, T (lower bed) = 548 K, P = 5.0 MPa, 1200 mL g -1 h-1. Catalyst loading: upper bed, 0.5 g GaZrO ; lower bed, mixture containing 0.5 g of MOR-8-C1 and 0.5 g of PC. The selectivity of organics was calculated on a molar carbon basis (CO free).\nComments 4: Compared to the GaZrO catalyst, both ZnZrO and InZrO exhibited significantly reduced catalytic activity and higher initial activity. Further in-depth discussion on this differential behavior is requested.\nResponse: We sincerely thank the reviewer for this insightful point. The higher initial activity of ZnZrO and InZrO compared to GaZrO is due to their higher amounts of oxygen vacancies, which facilitate CO activation. To clarify this, we have supplemented our study with XPS measurements of 2 the O 1s spectra for these catalysts after identical pretreatment conditions (Fig. R2). The analysis quantified the relative proportion of oxygen vacancies as 27.8% for GaZrO , 32.4% for ZnZrO , and 34.6% for InZrO . This trend in vacancy concentration directly correlates with and explains the trend in initial activity.\nThe differences in vacancy concentration can be understood from the different electronic and volumetric properties of the different guest cations (Fig. R3). While Ga and In exist in the +3 oxidation state, Zn is present as Zn2+. The larger charge difference between Zn²⁺ and the host Zr4+ cation promotes the generation of oxygen vacancies in ZnZrO . For the isovalent In³⁺ and Ga³⁺, the larger ionic radius of In3+ induces stronger lattice distortion within the ZrO matrix, thus facilitating vacancy formation 2 in InZrO . However, as the Reviewer pointed out, both ZnZrO and InZrO suffered from rapid deactivation and consequently exhibited inferior overall activity, despite of their high initial activity.\nAt the reaction temperature of 350 °C, ZnZrOₓ's instability was due to partial Zn volatilization and subsequent phase segregation. In the case of InZrO , excessive hydrocarbon (HCs) formation was detected, which promoted carbon deposition and consequently reduced stability. In contrast, GaZrO provides a more favorable balance between sufficient initial CO activation and long-term stability, 2 accounting for its superior overall catalytic performance.(Nature Catal. 2022, 5, 1038-1050; Angew. Chem. Int. Ed. 2021, 60, 17735-17743)\nThe corresponding discussions and modifications have been have been added to the revised manuscript: Line 11-16, Page8.\nFigure R2 O1s XPS of GaZrO , ZnZrO and InZrO .\nx x x\nFigure R3 XPS of Ga-2p in GaZrO , Zn-2p in ZnZrO and In-3d in InZrO .\nx x x\nComments 5: To enhance the hydrophobicity of the zeolite MOR surface, the integration of M2 and M3 strategies was employed, improving oxygenate selectivity under the M4 approach. Further explanation regarding this synergistic effect is requested. In addition, the effect of the introduction of carbon tubes should be expounded in the content.\nResponse: We thank the reviewer for raising this important point. The enhanced hydrophobicity of the M4 approach indeed stems from the synergy between external surface silanization (M2) and the incorporation of hydrophobic additives (M3). Macroscopically, this is confirmed by the significantly higher water contact angle of M4 compared to either M2 or M3 alone (Fig. 1c). The MD simulations were carried out to rationalize the respective contributions of the M2 and M3 modifications. As elaborately discussed in our manuscript, surface silanization (M2, Fig. 3b) effectively mitigates the axial movement of water molecules along the z-axis reduces their probability of entering the micropores (Supplementary Fig. 29). After silanization, D in the z-direction decreased dramatically s from 4.8 × 10-8 m2/s to 1.6 × 10-9 m2/s. In contrast, compared with the hydrophilic MOR model (Fig. 3f), the incorporation of hydrophobic additives (M3, Fig. 3g) accelerates water migration along the nanoslit surface in the x-y direction, thereby facilitating water’s quick escape into bulk gas phase and decreasing its residence time near the zeolite pore openings. To further highlight the superiority of the hydrophobic additives, we complemented a further modelling study by replacing the hydrophobic additives with hydrophobic MOR zeolite (Fig. R4). Compared with hydrophobic MOR model, the mean square displacement (MSD) of hydrophobic additive model nearly doubled along the x- and y- direction (Fig. R5 a, b), accompanied by an early appearance of water molecules in Region IV (Fig. R5c). The above results justify our addition of hydrophobic additives, despite the fact that surface silanization already enhances hydrophobicity. Surface silanization effectively mitigates the axial diffusion of water into the zeolite channels, while the external hydrophobic promoter facilitates rapid water escape into the bulk gas phase (Fig. 3e). The close synergy of the two approaches leads to the most enhanced hydrophobic effects in M4.\nThe corresponding discussions and modifications have been added to the revised manuscript:\nLine 9-19, Page12. Figures R4 and R5 have been added to the Supporting Information as Supplementary Fig. 32-33.\nAs for the role of carbon nanotubes, their inherently rough tube walls (SEM images, Supplementary Fig19) further increase surface hydrophobicity, consistent with literature reports. (Matter 2023, 6, 1992-2004) For example, after CNT incorporation, the contact angle of PTFE increased from 108° to 132°, confirming their contribution to the overall hydrophobic performance (Supplementary Fig. 18).\nThe corresponding discussions and modifications have been added to the revised manuscript:\nLine 20-24, Page7.\nFigure R4 Diffusion escape model of water molecules in hydrophobic MOR. Region I: hydrophilic MOR; Region III：hydrophobic MOR.\nFigure R5 Variations in mean-square displacement (MSD) of water molecules with molecular dynamics (MD) simulation time when region III consists of PC (a) and hydrophobic MOR(b) . (c) Variations in the number of water molecules in the four regions with MD simulation times.\nComments 6: For Strategy 4, no significant enhancement in CO conversion was observed. 2 Simultaneously, greater yields of target products were obtained, which reflected a strong reaction-pulling effect. Could this indicate that the hydrophobic modification strategy may adversely affect reaction diffusion and suppress feedstock conversion under reaction-pulling conditions? Further discussion on this phenomenon is requested.\nResponse: We thank the reviewer for this insightful comment. The reviewer is correct that Strategy 4 did not enhance overall CO conversion. This result is consistent with our reactor's dual-bed 2 configuration: the upstream GaZrO catalyst determines the maximum CO conversion, while the x 2 downstream zeolites (M1–M4) only transform the CH OH/DME intermediates produced upstream 3 (Supplementary Fig. 1). Therefore, the M4 modification on the downstream zeolite catalysts enhanced the carbonylation of the CH OH/DME intermediates into target products, rather than altering the 3 intrinsic activity of the GaZrO catalyst.\nx\nTo address the reviewer's concern regarding diffusion limitations, IGA tests using methyl acetate (the largest product molecule in this work) were complemented to determine the diffusion coefficient after hydrophobic modification. The diffusion coefficient only decreased slightly from 2.18 × 10⁻² to 1.12 × 10⁻² cm² s⁻¹ after trimethoxymethylsilane modification (Supplementary Fig. 8), confirming a relatively low impact on mass transport.\nThe corresponding discussions have been added to the revised manuscript: Line 9-14, Page 6.\nComments 7: Although good tandem conversion performance has been achieved on CO conversion 2 and oxygenate selectivity, more comparisons should be provided with relevant reports.\nResponse: We sincerely thank the reviewer for this valuable suggestion. As our work represents the first demonstration of a tandem catalytic strategy for CO₂ conversion into oxygenates such as methyl acetate, acetic acid, and ethanol, no directly comparable catalytic systems with identical mechanistic pathways are available in the literature.\nTherefore, we referred to studies targeting similar products (e.g., ethanol) but following distinct catalytic pathways, with reactions conducted either in fixed-bed or batch reactors, to provide a more comprehensive comparison. We acknowledge that, due to the mechanistic differences, our system may not exhibit advantages across all parameters. Nevertheless, the tandem catalysis strategy combining binary oxides with zeolites enables facile control over both product distribution and selectivity. These comparisons, however, underscore the novelty of our approach and highlight its potential value as a viable route for CO valorization (Table R2).\nLine 6-10, Page16. Table R2 have been added to the Supporting Information as Supplementary Table 4.\nTable R2 Catalysis Performance of Some Related Catalysts for CO Hydrogenation.\nCatalyst T (K) P(MPa) Reference\n(mL g -1 h-1) (%) calculation (C-mol%) (mg g -1 h-1)\ncat cat\nGZO+MOR 623+558 5.0 1200 27.5 MA+AA 78.8 25.9\nThis work\nGZO+MOR+CZA 623+558 5.0 1050 27.5 EtOH 50.5 13.1\n1Pd Ce@Si16 523 3.0 3000 5.9 EtOH 98.7 116 1\n3 4\nRh-VO /MCM-4 523 3 6000 12.1 EtOH 24.1 0.48 3\nx\nIr -In O c 473 6 EtOH 99.7 0.46 4\n1 2 3\nPd Cu/P25 c 473 3.2 EtOH 92 0.53 5\n1 x\nLi-Rh/SiO 513 5 6000 7.0 EtOH 18.4 16.7 7\nCoAlO c 413 4.0 - - EtOH 92.1 0.44 9\nx\nCoMoS 613 10.4 0.43d 32 EtOH 12.9 3.5 10\nCo/La Ga O 543 3.0 3000 9.6 EtOH 23.3 10.1 11\n4 2 9\nCu@Na-Beta 573 1.3 12000 7.9 EtOH 100 258 12\nCs/Cu-Fe-Zn 603 5 4500 36.6 C OH 13.7 52 13\n2+\na. Selectivity calculated with CO (CO free); b. Yield calculation is based on the compounds listed above. c, Reactions were carried out in a batch reactor. d, GHSV: g g -1 h-1\ncat\nComments 8: More explanation about the formation processes and regulation mechanisms for different oxygen-containing compounds should be provided.\nResponse: We sincerely thank the reviewer for this insightful comment. In our tandem system, CO is 2 first hydrogenated on GaZrOx in the upstream reactor to CH₃OH/DME and CO. These intermediates then undergo carbonylation on the modified MOR zeolite in the downstream reactor to form acetic acid (AA) and methyl acetate (MA).\nTo elucidate the reaction pathways over the upstream GaZrO catalyst, we conducted in situ x DRIFTS experiments. Figure R6 shows the spectra for CO hydrogenation over GaZrO (0.5) at 350 °C 2 x and 2 MPa. The appearance of a band at 1467 cm⁻¹, assigned to surface carbonates, indicates CO₂ adsorption on oxygen vacancies. With time, characteristic bands emerged for formate (HCOO* at 1576 and 1386 cm⁻¹) and surface methoxy (CH O* at 1130 and 1041 cm⁻¹) species. This variation confirms 3 that CO is activated at oxygen vacancies, generating HCOO* and CH O* intermediates that are 2 3 hydrogenated to CH₃OH/DME. The concurrent formation of CO is verified by features at 2100–2170 cm⁻¹.\nThe carbonylation mechanism in the downstream reactor was also followed by in situ DRIFTS experiments (Fig. R7), using a mixture of DME/CO as the reactants. Upon introducing CO/DME, absorption bands of gaseous DME (2800–3000 cm⁻¹) and CO (2000–2200 cm⁻¹) were observed. After 10 min reaction, a new band at 1740 cm⁻¹ appeared, assignable to the C=O stretching of methyl acetate, along with bands at 1160–1180 cm⁻¹ (C–C stretching in oxygenates) and 1465/1390 cm⁻¹ (CH₃ bending). To identify intermediates, the feed was switched to Ar purge. After 10 min, new signals at 2946 and 2838 cm⁻¹ emerged, which can be attributed to surface methoxy groups (CH₃O*), confirming that DME undergoes activation at Brønsted acid sites to form methoxy species, which subsequently react with CO to generate acetyl intermediates and ultimately methyl acetate.\nRegarding regulation mechanisms, we demonstrate that tuning the composition of the CO₂ hydrogenation catalyst and the reaction conditions allows control over the DME/CH3OH/CO composition from the upstream reactor, while hydrophobic modification of zeolites modulates the AA/MA ratio and selectivity. As shown in Supplementary Tables 1 and 2, modifying the catalyst (M1– M4) shifts the oxygenate distribution: increased hydrophobicity promotes DME conversion to MA while suppressing AA formation by reducing water inhibition. These results demonstrate that product selectivity can be effectively modulated within this tandem framework. Beyond MA and AA, we also demonstrated ethanol production via hydrogenation (Supplementary Fig. 37), indicating the system’s potential to deliver a broader range of oxygenates through adjustments in catalyst composition and bed configuration. Furthermore, replacing GaZrO with ZnZrO enhanced the methanol selectivity in the x x upstream reactor, and consequently an improved AA selectivity for our overall tandem catalysis system. (Table R1).\nThe corresponding discussions and modifications have been added to the revised manuscript: Line 16-22, Page8. Figure R6-8 have been added to the Supporting Information as Supplementary Fig. 26- 28.\nFigure R6 In situ DRIFTS spectra of CO hydrogenation on GaZrO . The spectra were collected 2 x every 3 min up to 30min under reaction conditions of 350 °C and 2 MPa.\nFigure R7 In situ DRIFTS spectra of the DME carbonylation reaction on MOR-8-C1. The spectra were collected under reaction conditions of 553 K and 2 MPa. Following the introduction of the reactant gas for 10 min, spectra were collected every 3mins up to 30 min. Subsequently, the feed was switched to argon(Ar), and spectra were collected during a 20 min purge.\nFigure R8 Schematic illustration of the catalytic mechanism of the tandem system (GaZrO x + MOR- 8-C1@PC).\nReviewer 2\nThe manuscript entitled “CO Hydrogenation to Acetic Acid and Methyl Acetate via Tandem 2 Catalysis” presents an innovative and well-structured approach to convert CO into value-added C 2 2+ oxygenates through a dual-bed tandem catalytic system. The upstream GaZrO catalyst efficiently x converts CO to a CO/CH OH/DME intermediate gas mixture, while the downstream hydrophobically 2 3 modified MOR zeolite (MOR-8-C1@PC) enables selective carbonylation to methyl acetate and acetic acid. The hydrophobic engineering strategy to mitigate water inhibition is scientifically sound and well-substantiated through molecular dynamics simulations and surface analyses. Overall, the study provides substantial insight into the integration of tandem catalysis and water management for efficient CO utilization. However, to further clarify and strengthen the manuscript, the following minor 2 revisions and scientific questions are suggested:\nResponse: We appreciate the reviewer for the positive comments and suggestions for improving this manuscript.\nComments 1.The MD simulations show reduced water ingress into hydrophobic MOR channels, but how does hydrophobicity precisely preserve active sites? Are there experimental data (e.g., in situ IR spectroscopy) confirming reduced water adsorption on BAS in 8-MR pockets? Why does MOR-8-C1 (CA=121°) outperform MOR-8-C3 (CA=133°) in stability despite lower hydrophobicity? Is this solely due to thermal stability, or do methyl groups offer unique steric/electronic effects?\nResponse: We appreciate the reviewer’s insightful questions. For the MOR-catalyzed carbonylation reaction, the presence of water is known to be detrimental, as previously reported (J. Am. Chem. Soc. 2008, 130, 16316–16323). The hydrophobic modification introduces surface methyl groups that effectively reduce the probability of water ingress into the channels, thereby preserving the catalytic activity of Brønsted acid (BAS) sites.\nTo directly validate this protective effect, we conducted in situ IR experiments. The experimental setup is shown in Fig. R9: Water was stored in a temperature-controlled reservoir, carried into the in situ cell by Ar, and regulated by a back-pressure valve. The catalysts were first dehydrated at 300 °C to expose BAS, and subsequently exposed to saturated water vapor (45 °C) at 270 °C and 2 MPa. Spectra were collected every 3 min. For unmodified MOR-8, the characteristic BAS band at 3582 cm⁻¹ decreased significantly upon water introduction (0–30 min), while new bands at 3650 cm⁻¹ and 3480 cm⁻¹ emerged, corresponding to H O-BAS interactions and hydrogen-bonded water species (Fig. R10). 2 By contrast, in hydrophobically modified MOR-8-C1, the 3582 cm⁻¹ BAS band showed only a minor decrease under identical conditions, confirming that hydrophobic modification mitigates water- induced deactivation (Fig. R11).\nRegarding the difference between MOR-8-C1 and MOR-8-C3, longer alkyl chains indeed impart stronger hydrophobicity, as they offer more nonpolar surface coverage (Journal of the American Chemical Society 2023, 145, 26052-26060). However, extended chains also contain more β-H atoms, which makes the grafted organosilane prone to C–C bond cleavage, thereby compromising thermal stability (Microporous and Mesoporous Materials 2022, 344, 112217). Our systematic screening indicated that MOR-8-C1 exhibited the best thermal robustness, which is critical for long-term operation. The surface methyl groups thus function solely as nonpolar shielding moieties without introducing additional steric or electronic effects.\nFigure R9 In situ infrared spectroscopy setup.\nFigure R10. In situ DRIFTS spectra of MOR-8 under water-feeding conditions. The experiments were conducted at T = 543 K and P = 2 MPa. After dehydration at 573 K, water vapor saturated at 45 °C was introduced into the in situ cell with Ar as carrier gas, and spectra were collected every 3 min.\nFigure R11 In situ DRIFTS spectra of MOR-8-C1 under water-feeding conditions. The experimental test conditions are the same as those in Fig. 10.\nComments 2.GaZrO achieves a CO/(DME+CH OH) ratio of ~4. How was this ratio identified as x 3 optimal? What happens if the ratio deviates (e.g., higher CO or DME)? Why does GaZrO x show superior stability over InZrO /ZnZrO ? Is Ga doping critical for oxygen vacancy stability?\nx x 3+\nResponse：We sincerely thank the reviewer for this constructive question. The identification of the optimal CO/(DME+CH₃OH) ratio was based on systematic experiments under controlled conditions.\nAs shown in Supplementary Fig. 23, we first examined the product distribution of GaZrO at different x reaction temperatures in a single-reactor setup, and determined the corresponding outlet CO/(DME+CH₃OH) ratios. In the tandem system, we subsequently fixed the MOR temperature and tuned the GaZrOx temperature to adjust the upstream CO/(DME+CH₃OH) ratio, while monitoring the downstream selectivity and yield of MA and AA. As illustrated in Fig. 4a, across 25 experimental runs, the condition with a CO/(DME+CH₃OH) ratio of ~4 consistently matched the MOR operation window (553–563 K) and delivered the highest MA+AA selectivity.\nWhen the CO fraction increases beyond this ratio (e.g., at 673 K), the GaZrO reaction x predominantly produces CO with limited DME and CH₃OH. Although carbonylation proceeds efficiently under these conditions, the insufficient availability of C1 oxygenates leads to lower overall yields of MA and AA. Conversely, when the DME fraction is higher, the CO/(DME+CH₃OH) ratio decreases, and the downstream MOR is unable to fully convert the excess DME, resulting in residual unreacted intermediates. This balance therefore reflects a thermodynamically and kinetically tuned compromise between the two catalyst stages, rather than an arbitrarily chosen value.\nRegarding the superior stability of GaZrO compared with InZrO and ZnZrO , we attribute this x x x to the intrinsic differences in cation behavior under high-temperature (350 °C) reaction conditions. For ZnZrO , Zn²⁺ tends to volatilize and segregate, causing structural degradation and stability loss. In the x case of InZrO , significant hydrocarbon byproducts are formed at 350 °C, which promote coke x deposition and deactivate the catalyst. Similar stability issues have been reported previously for these systems (Nature Catalysis 2022, 5, 1038-1050). In contrast, Ga³⁺ doping in the ZrO matrix plays an 2 important role, not only generating oxygen vacancies in adjacent positions, but also ensuring the stability under reaction conditions.\nComments 3.Hydrocarbon selectivity rises at >563 K (Fig. 4a). Is this purely due to temperature- dependent MTH activation?\nResponse: We thank the reviewer for this insightful comment. The observed increase in hydrocarbon selectivity at temperatures above 563 K arises from two factors. First, at a fixed reaction temperature on GaZrO , the higher hydrocarbon formation is attributed to the partial exposure of Brønsted acid x sites in the 12MR, which enhances the DME/CH₃OH-to-hydrocarbon (MTH) conversion. Second, when the reaction temperature on GaZrO itself increases, additional hydrocarbons are generated on x the catalyst surface; these species, being unconverted in the downstream, directly contribute to the observed increase in the final hydrocarbon selectivity.\nComments 4.1H MAS NMR shows unchanged Si(OH)Al peaks post-silanization, but does reaction- induced hydrolysis regenerate surface hydroxyls? Provide 29Si NMR data after catalysis to confirm silane stability.\nResponse: We thank the reviewer for raising this important point. We have supplemented the study with the corresponding 29Si NMR data, as shown in Fig. R12. After catalysis, the resonances at −106 and −112 ppm, assigned to Q³ (Si(OH)(OSi)₃) and Q⁴ (Si(OSi)₄), remain essentially unchanged, indicating that no additional silanol groups are generated. At the same time, the signal at −66 ppm corresponding to Si–CH₃ species is well preserved and even slightly enhanced, confirming that the surface retains its silane coverage. By contrast, the signal at −66 ppm assigned to Si–O–CH₃ species decreases, suggesting that under reaction conditions partial Si–O bond cleavage may occur, promoting transformation into the more stable Si–CH₃ configuration. This dynamic adjustment of the surface silane layer appears unavoidable, yet the overall hydrophobic character of the material is preserved. Indeed, the water contact angle of the spent catalyst was measured to be ~103°, indicating that the material retains a high degree of hydrophobicity after reaction.(Fig. R13)\nFigure R12 29Si MAS NMR spectra of MOR-C1 and MOR-C1-AR.\nFigure R13 Water contact angle of MOR-8-C1 after reaction.\nComments 5.What is the long-term impact of accumulated water or possible coke formation on the structural and catalytic stability of MOR-8-C1@PC, and how can it be mitigated in industrial settings?\nResponse: We thank the reviewer for raising this critical point. Both carbon deposition and water accumulation can indeed exert pronounced effects on catalyst structure and performance. Progressive coke formation accelerates catalyst deactivation and, under severe conditions, may even block the zeolite channels. Water accumulation, on the other hand, increases the local partial pressure of H₂O, which facilitates competitive adsorption of DME and water on the BASs of MOR, thereby significantly reducing catalytic activity. From an industrial perspective, these issues can be mitigated by operating at a moderately reduced temperature to stabilize the surface modifiers and suppress coke formation. In practice, the introduction of water-removal units into the process flow can effectively maintain a low- moisture reaction environment. Alternatively, controlled co-feeding of CO can lower the net water yield during tandem catalysis. Importantly, our study demonstrates a modification strategy that maintains high catalytic performance even under water-rich environments, thereby providing a robust route for extending the applicability of the catalyst to industrial processes.\nComments 6.How does the hydrophobic modification affect the adsorption/desorption dynamics of key intermediates (e.g., DME, CH OH, CO) beyond just water diffusion, and can this influence product 3 distribution or selectivity under different reaction conditions?\nResponse: Thank you for this constructive comment. The hydrophobic modification was performed on the external surface of MOR by introducing nonpolar groups such as methyl. As CO is a small, nonpolar molecule, its diffusion is unaffected by this modification. Similarly, dimethyl ether (DME), which contains two methyl groups, shows negligible change in adsorption or desorption behavior upon modification.\nFor methanol, which possesses a polar hydroxyl group, contact angle measurements on the modified MOR-8-C1 indicate a value of approximately 10° (Supplementary Fig. 22), suggesting that methanol adsorption is also essentially unchanged. Beyond electronic effects, diffusion effects were considered: adsorption tests using the largest molecular probe, MA, showed that MOR-8-C1 slightly hinders MA diffusion (Supplementary Fig. 8), but this does not impede the reaction. Therefore, smaller molecules such as methanol and DME experience no diffusion limitation. Taken together, the hydrophobic modification primarily shields polar water molecules, while having negligible impact on the adsorption/desorption dynamics of other reactant molecules, and thus does not significantly affect product distribution or selectivity.\nComments 7.Abbreviations: Define all acronyms (e.g., PC, MTH, STY) upon first use.\nResponse: We thank the reviewer for pointing this out. All abbreviations, including PC, MTH, and STY, have now been defined upon first use in the main text.\nComments 8.The phrase “facilitating he subsequent” in the Abstract contains a typo. Revise to “facilitating the subsequent” for grammatical accuracy.\"\nResponse: We thank the reviewer for this careful suggestion and have incorporated the corresponding changes in the main text.\nResponses to the reviewers’ comments\nReviewer #1 (Remarks to the Author):\nThe authors have provided thoughtful responses to the issues we raised early, and this manuscript is suggeated to be accepted.\nResponse: Thank you very much. We sincerely appreciate your insightful comments and guidance on this study.\nReviewer #2 (Remarks to the Author):\nThe authors have thoroughly addressed all of my concerns. They have provided detailed responses and made corresponding revisions to the manuscript. I am pleased to recommend the publication of this manuscript in Nat. Commun.\nResponse: Thank you very much. Very much appreciated it!"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have provided thoughtful responses to the issues we raised early, and this manuscript is suggeated to be accepted.", "reviewer_2": "The authors have thoroughly addressed all of my concerns. They have provided detailed responses and made corresponding revisions to the manuscript. I am pleased to recommend the publication of this manuscript in Nat. Commun."}, "author_response": ""}]

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