CO_2 hydrogenation toward toluene methylation for selective para-xylene synthesis

Domain: physics
Published: 2025-12-24
Editorial outcome: minor_revision

Abstract
The selective production of high-value para-xylene (PX) through CO2 hydrogenation-coupled toluene methylation represents a promising strategy for sustainable carbon utilization. Herein, we report a ZnZrO x & HMCM-22 tandem catalyst that integrates CO2 hydrogenation with shape-selective toluene methylation. The optimized catalyst achieves 91% xylene selectivity with 70.4% PX dominance at 10.3% toluene conversion. Controlled tetraethyl orthosilicate (TEOS) deposition passivates Brønsted acid sites in HMCM-22 supercages, suppressing toluene disproportionation while preserving methylation activity in sinusoidal channels. Mechanistic studies reveal that formaldehyde species, generated on ZnZrO x and transferred to HMCM-22, act as kinetically favored methylating intermediates, enabling direct coupling between CO2 hydrogenation and toluene methylation. This pathway outperforms conventional methanol-mediated routes in both activity and selectivity. Our work establishes a dual-optimization strategy—zeolite microenvironment engineering and reactive intermediate control—offering mechanistic insights and design principles for high-performance tandem catalysis in carbon recycling aromatic synthesis.

Introduction
Carbon dioxide, as an abundant C1 feedstock, presents a great opportunity for sustainable chemical synthesis when coupled with green hydrogen derived from renewable energy. Recent advances have demonstrated the feasibility of converting CO2/H2 mixtures into value-added products including methanol1,2, light olefins3–6, and aromatics7–9. Although direct CO2 hydrogenation to aromatics has been reported using tandem catalysis, the para-xylene (PX) selectivity within aromatic products remains suboptimal. This limitation contrasts sharply with the industrial demand for PX—a precursor for polyethylene terephthalate (PET) manufacturing—which currently relies predominantly on fossil-based routes such as naphtha cracking and methanol-mediated toluene alkylation. These considerations highlight the urgency to develop efficient strategies for synthesizing PX through CO2-derived methylating agents with toluene, thereby enabling PX production via a two-step CO2/H2 pathway.

Conventional toluene methylation processes employing methanol confront intrinsic thermodynamic limitations when implemented over H-ZSM-5 zeolites at 400–500°C. Under these harsh thermal conditions, the concurrent activation of methanol-to-olefins (MTO) side reactions precipitates a cascade of operational challenges: competitive methanol consumption by olefin synthesis pathways leads to methylating agent depletion, while concomitant carbon deposition accelerates catalyst deactivation through pore blockage and active site coverage10,11. Furthermore, the kinetic rivalry between methylation and MTO pathways severely compromises reaction efficiency. This tripartite interplay of resource misallocation, catalyst instability, and suppressed target reaction kinetics collectively necessitates the development of catalytic architectures capable of circumventing methanol intermediation under thermodynamically favorable conditions.

Recent developments in tandem catalysis reveal promising alternatives through in situ-generated oxygenate intermediates. Oxide-zeolite composites (e.g., oxide: ZnZrO x, ZnGaO4, In2O3-ZrO2, ZnCrOx; zeolites: SAPO-34, SSZ-13, H-ZSM-5)12–16 enable CO2 hydrogenation to lower olefins and aromatics via CH x O intermediates through formate-mediated pathways. Notably, these systems operate at significantly lower temperatures than conventional methanol-based processes, as CH x O intermediates exhibit lower activation barriers compared to free methanol. Our previous work further demonstrated that methanol synthesis in tandem systems is thermodynamically driven by downstream olefin formation—a critical coupling mechanism that facilitates intermediate-directed catalysis4. These findings suggest that judicious selection of reactive methylating intermediates could decouple methylation kinetics from MTO side reactions.

Emerging CO2 hydrogenation-coupled toluene methylation strategies underscore the pivotal influence of zeolite architectural design on product selectivity17–20. Whereas H-ZSM-5-based systems deliver moderate xylene yields, their inherent structural constraints—notably unidirectional channel geometry and pronounced Brønsted acidity—predispose the system toward competing toluene disproportionation and xylene isomerization pathways21,22. In contrast, MCM-22 zeolite demonstrates distinctive structural merits for shape-selective alkylation, characterized by sinusoidal 10-membered ring (10-MR) channels that impose steric restrictions on bulkier isomers, coupled with chemically tunable supercage configurations enabling precise acid site modulation. The spatial isolation of active sites within this architecture further suppresses disproportionation side reactions. These collective structural attributes establish MCM-22 as an exemplary scaffold for constructing tailored methylation microenvironments with enhanced selectivity23,24.

In this work, we develop a bifunctional ZnZrO x & HMCM-22 (ZZO & M-22) tandem catalyst that synergistically integrates CO2 hydrogenation and shape-selective methylation. Through strategic silica deposition in MCM-22 supercages, we selectively passivate Brønsted acid sites responsible for toluene disproportionation while preserving methylation-active sites in sinusoidal channels. Experimental results, advanced characterization via in situ synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) and density functional theory (DFT) calculations identify formaldehyde-like CH2O* as the dominant methylating intermediate, exhibiting a 0.86 eV lower activation barrier compared to methanol-mediated pathways. The optimized catalyst achieves 91% xylene selectivity with 70.4% PX dominance among isomers at 10.3% toluene conversion. This study establishes a dual-optimization strategy combining zeolite microenvironment engineering with reactive intermediate control, providing a blueprint for developing industrially viable CO2-to-aromatics systems.

Results
Catalytic performance
The catalytic performance of tandem catalyst systems was systematically investigated in a fixed-bed reactor under controlled reaction conditions. Temperature-dependent studies show a monotonic increase in toluene conversion from 3% to 32% with simultaneous attenuation of PX selectivity with increasing reaction temperature from 280 to 400°C (Fig. 1a, and Supplementary Fig. 1a). Thermodynamic redistribution of xylene isomers favored meta-xylene (MX) formation, accompanied by decreasing benzene selectivity and progressive accumulation of 1,2,4-trimethylbenzene (TMB) with increasing reaction temperature. This temperature-response profile suggests competing mechanisms: enhanced alkylation kinetics promoting active methyl species-toluene coupling at elevated temperatures, versus thermally activated processes including xylene isomerization to MX and over-methylation to TMB. Comparative evaluation of individual components in the ZZO & M-22 tandem system demonstrated distinct functionalities ZZO exclusively mediated CO2-to-methanol conversion while M-22 remained catalytically inert in isolation for CO2 hydrogenation (Fig. 1b). The combined system achieved integrated CO2 hydrogenation and toluene methylation without generating aromatization byproducts, with control experiments confirming toluene disproportionation as the dominant pathway on isolated M-22 with toluene as feedstocks.

Modification of M-22 via tetraethyl orthosilicate (TEOS) deposition induced three critical effects: pore architecture modulation favoring PX diffusion, passivation of external acidic sites, and suppression of disproportionation pathways. Progressive TEOS deposition cycles (1–4 cycles) enhanced PX selectivity from 43% to 64% while suppressing benzene selectivity from 10% to <1%, though with concomitant TMB accumulation (9%) due to restricted PX diffusion enabling secondary methylation (Fig. 1c). Five deposition cycles induced pore blockage, manifesting as activity loss with toluene conversion decreasing from 14% to 7%, establishing 4-cycle modification (ZZO & 4Si/M-22) as the optimal configuration. The ZZO & 4Si/M-22 catalyst achieves 91% xylene selectivity with 70.4% PX dominance among isomers at 10.3% toluene conversion. Compared to other catalyst systems, the ZZO & 4Si/M-22 can obtain a high xylene selectivity while achieving a high PX selectivity, and can completely inhibit the occurrence of toluene disproportionation side reactions (Supplementary Table 1). And the ZZO & 4Si/M-22 achieved a CO2 conversion of 4.2% with a CO selectivity of 22% at 320°C (Supplementary Fig. 1b). Long-term stability tests confirmed sustained performance (>120 h) with undiminished PX selectivity (Fig. 1d).

Process parameter optimization revealed pressure-dependent behavior with elevated pressure from 1 to 6 MPa, which enhances toluene conversion from 5% to 12% but promotes TMB formation with selectivity increasing from 4% to 12% through PX retention-induced over-methylation (Supplementary Fig. 2a). Gas hourly space velocity (GHSV) studies identified 12,000 mL g−1 h−1 as optimal, balancing toluene conversion (10.3%) and selectivity (PX 64%, TMB 9%), with lower GHSV to 6,000 mL g−1 h−1 inducing disproportionation pathways (Supplementary Fig. 2b). Weight hourly space velocity (WHSV) adjustments demonstrated an intrinsic activity-selectivity trade-off: increasing WHSV from 1 to 3 h−1 improved PX selectivity from 64 to 80% but incurred 80% toluene conversion loss with decreasing from 10.3% to 2%, while simultaneously suppressing TMB formation (Supplementary Fig. 2c). These systematic investigations establish the ZZO & 4Si/M-22 system as an effective tandem catalyst for selective PX production, achieving precise control through synergistic component integration, strategic pore engineering, and optimized reaction parameters.

Structure characterization of the tandem catalysts
The ZZO solid solution oxide catalyst was synthesized through 20 mol% ZnO doping into ZrO2. X-ray diffraction analysis confirms that the resultant ZZO solid solution adopts a characteristic tetragonal crystal structure, as evidenced by the diffraction pattern in Fig. 2a. As shown in the transmission electron microscopy (TEM) image (Fig. 2b), the ZZO material exhibits an nanoparticle morphology with uniform dimensions. Elemental mapping analysis further demonstrates the homogeneous distribution of Zn atoms within the ZrO2 matrix (Fig. 2c), verifying the successful formation of a solid solution structure. The M-22 zeolite component displays well-defined nanosheet morphology, as observed in Fig. 2d. These crystalline sheets exhibit thicknesses of approximately 28 nm and lateral dimensions ranging from 300 to 500 nm. In the ZZO & M-22 tandem catalyst system, scanning electron microscopy (SEM) characterization reveals that ZZO nanoparticles are uniformly dispersed across the M-22 nanosheet surfaces (Fig. 2e). A critical observation from the high-resolution SEM image is the preservation of distinct structural features for both components, indicating that the interfacial interaction between ZZO and M-22 remains primarily physical rather than chemical. Additional structural validation is provided in Supplementary Fig. 3a, b, which present SEM images of the 4Si/M-22 modified sample. The morphology of M-22 nanosheets remains essentially unchanged after TEOS treatment, confirming that the structural integrity of the zeolite framework is maintained during the modification process. And the specific BET surface area, micropore area, external area and pore volume of M-22 all decreased, while the pore diameter remained almost unchanged. (Supplementary Table 2). This indicates that TEOS does not deposit extensively within the internal pores of the M-22, but predominantly at the entrances of all channels and on the external surface, thereby enhancing the shape selectivity of M-22.

Surface chemical properties of the tandem catalysts
CO2 temperature-programmed desorption (CO2-TPD) analysis reveals distinct adsorption behaviors between components (Supplementary Fig. 4a). The ZZO solid solution demonstrates sustained CO2 adsorption up to 500°C, while the zeolite component exhibits negligible CO2 adsorption capacity. Quantitative evaluation of basic sites confirms that the physical integration of zeolite in the tandem catalyst exerts minimal influence on CO2 adsorption performance of ZZO (Supplementary Table 3). The superior PX selectivity observed in ZZO & 4Si/M-22 tandem catalysts highlight the critical role of TEOS modification. This performance enhancement correlates with the decisive influence of zeolite acid properties (both quantity and type) on PX selectivity. To elucidate this relationship, comprehensive acid characterization was conducted using NH3-TPD, pyridine Fourier-transform infrared (Py-FTIR) spectroscopy, 1H MAS NMR, and 27Al MAS NMR. NH3-TPD profiles (Supplementary Fig. 4b) exhibit two characteristic desorption regions: weak acid sites (50–250°C) and strong acid sites (250–500°C). As shown in Supplementary Table 4, the total acid content of the ZZO & M-22 tandem catalyst closely matches the arithmetic sum of individual components, confirming that physical mixing preserves the intrinsic acidity of both ZZO and M-22.

Systematic investigation of acid evolution for M-22 under successive TEOS modifications reveals progressive acid suppression (Fig. 3a). Notably, both weak and strong acid contents stabilize after four modification cycles, indicating saturation of surface passivation. Py-FTIR analysis identifies characteristic bands at 1540 cm−1 (Brønsted acid sites) and 1440 cm−1 (Lewis acid sites), with quantitative analysis following established protocols (Fig. 3b)25. Supplementary Table 5 demonstrates a marked reduction in both acid types with increasing modifications, particularly the near-complete elimination of Brønsted acidity after four cycles. This evidences TEOS-mediated encapsulation of acid sites within external surface pockets of M-22. 1H MAS NMR spectra (Fig. 3c) provide further validation of TEOS-induced acid modulation. The diminishing signal at 1.8 ppm (assigned to external silanol groups, Si-OH26) and attenuated resonance at 3.9 ppm (bridging hydroxyls, Si(OH)Al27,28) correlate with modification cycles. Concurrent chemical shift variations in the 3.9 ppm band reinforce the role of TEOS in external surface acid passivation, as quantified in Supplementary Table 6.

Considering the intrinsic link between Brønsted acidity and framework aluminum (Al F) in zeolites (Supplementary Fig. 5), 27Al MAS NMR analysis was employed to probe Al speciation (Fig. 3d). Distinct resonances at 0 ppm (extra-framework Al EF, 6-coordinated) and 56 ppm (framework Al F, 4-coordinated) were identified29. Deconvolution of the 56 ppm band resolves three components: 50 ppm (T6+T7 sites), 56 ppm (T1+T3+T4+T5+T8 sites), and 61 ppm (T2 sites)30. Structural analysis localizes T2 and T1+T3 sites in supercages/surface pockets, T4 in inaccessible framework positions, T5 in sinusoidal channels with unstable and unfavorable for siting Al, and T8 in the sinusoidal channels connected to the bottom of the supercages, with thermodynamically stable bridging hydroxyls31–33, with T6+T7 residing on external surfaces34. Progressive TEOS modification induces three critical changes: gradual attenuation of the Al EF signal at 0 ppm; systematic reduction of overall Al F intensity at 56 ppm; selective disappearance of the 61 ppm component (T2 supercage sites) upon deconvolution (Supplementary Table 7, Supplementary Fig. 6a).

These observations suggest TEOS not only passivates external surface pocket acid sites but also modulates supercage acidity. This aligns with literature demonstrating toluene disproportionation (a benzene-forming side reaction) predominantly occurs in MWW-type supercages35,36. Consistent with Fig. 1c, reduced benzene selectivity parallels TEOS-induced supercage deactivation. Complementary 27Al MAS NMR analysis of pre-coked M-22 confirms analogous attenuation of 61 ppm signals (Supplementary Fig. 6b, c), validating efficacy of TEOS in suppressing toluene disproportionation through targeted acid site passivation.

Unraveling the key formaldehyde intermediates species and the reaction mechanism
The reaction mechanism was investigated through multiple complementary techniques. In situ DRIFTS analysis of ZZO during CO2 hydrogenation revealed formate (HCOO*) and methoxy (HCO*) intermediates, with characteristic vibrations at 1595/1370 cm−1 (OCO stretching) and 2931/2824 cm−1 (C-H modes), respectively (Supplementary Fig. 7a). Isotopic substitution (CO2 + D2) caused rapid depletion of HCOO*/H3CO* signals within 5 min, replaced by transient DCOO* (2165 cm−1) and HD2CO* (2050 cm−1) species, confirming the formate pathway dominates CO2 hydrogenation over ZZO (Supplementary Fig. 7b)1. Isotope-switching (13CO2) experiments confirmed CO2 incorporation into products through 15 cm−1 redshift (2880→2865 cm−1) in methyl C-H vibrations and intensified aromatic signals (Fig. 4a)21,37,38. Compared to ZZO, ZZO & 4Si/M-22 showed selective disappearance of H3CO* species, while the HCOO* species still retain suggesting intermediate derived from formate species transfer to the zeolite component, thereby involving the formation of PX (Fig. 4b).

Quasi in situ 13C NMR revealed surface species evolution (Fig. 4c): ZZO in 13CO2 + H2 showed formates (169 ppm), methoxy (52 ppm), and methanol (49 ppm)39,40 while ZZO & 4Si/M-22 in 13CO2 + H2 + C7H8 exhibited depleted CH x O signals alongside emerging xylene and trimethylbenzene signatures (20.9 ppm and 19.5 ppm)41,42. 1H-13C 2D heteronuclear correlation (HETCOR) NMR correlations confirmed these assignments formate (169/8.4 ppm), methoxy (52/3.7 ppm), and methanol (48/3.2 ppm) in ZZO (Fig. 4d), transitioning to PX and 1,2,4-trimethylbenzene signals (20.9/2.4 ppm and 19.5/1.84 ppm) in the tandem system (Fig. 4e). Collectively, these results demonstrate CH x O* identified as the key intermediate enabling selective PX formation through spatial proximity effects in the tandem catalyst system, and the depletion of CH x O species promotes formate conversion.

To further identify the CH x O intermediates and their migration behavior from ZZO to MCM-22, controlled experiments were performed (Fig. 4f, 4g). Significantly, side product—specifically 2-(m-methylbenzyl)toluene (2MMBT), 4-(p-methylbenzyl)toluene (4PMBT), and 2-(p-methylbenzyl)toluene (2PMBT)—were detected with selectivity of 3% during CO2 hydrogenation coupled with toluene over the ZZO & M-22 tandem catalyst (Fig. 4f, top panel; Supplementary Fig. 8a). This observation suggests that CH x O species, generated from CO2 hydrogenation, react with toluene, initially forming methylbenzyl alcohol intermediates. These intermediates then undergo further reactions with toluene to produce the observed side products. Crucially, when formaldehyde (CH2O) was used as a methylating agent, replacing CO2/H2, identical side products were formed. In contrast, methanol, as a methylating agent, did not lead to the formation of these compounds. Furthermore, formaldehyde exhibited more favorable reaction kinetics compared to methanol, facilitating PX formation at lower temperatures and achieving higher PX selectivity (Supplementary Fig. 8b). In situ SVUV-PIMS analysis enabled the identification of key intermediates through their characteristic ionization thresholds (Fig. 4h, i): m/z = 30 (H2CO, 10.88 eV) and m/z = 32 (CH3OH, 10.85 eV), which are consistent with values reported in the literature43,44. These findings confirm that CH x O* are generated over ZZO, and that formaldehyde—not methanol—acts as the alkylating agent formed from CO2 + H2 over the ZZO & M-22 tandem catalyst. Reaction kinetics further support the formation of CH x O* species formed over ZZO from CO2 hydrogenation, with methanol being produced over ZZO (Fig. 1b). To confirm that alkylation occurs exclusively over MCM-22 and not ZZO, the reaction of CO2 + H2 with toluene was carried out over isolated ZZO catalyst (Supplementary Fig. 9). This experiment unequivocally demonstrated that alkylation requires the presence of the MCM-22 component. These results indicate the CH x O* species, formed over ZZO, can migrate to M-22 site to contribute to the formation of PX.

Based on the results presented above, the following reaction pathway is proposed, as illustrated in Fig. 4j. Initially, CO₂ and H₂ are adsorbed and activated on the ZZO component, leading to the formation of HCOO* species. These HCOO* species then undergo hydrogenation to produce CH x O* intermediates. The CH x O* intermediates subsequently migrate to the MCM-22 zeolite, where they react with toluene to form methylbenzyl alcohol, with para-methylbenzyl alcohol being the dominant product. The para-methylbenzyl alcohol then primarily undergoes dehydration, followed by hydrogenation, to yield the target product, PX. In parallel, some of the methylbenzyl alcohol can react with an additional toluene molecule, resulting in the formation of side products—2MMBT, 4PMBT, and 2PMBT.

DFT calculations on the reaction pathway
The experimental findings collectively suggest that CH x O* serves as the critical intermediate enabling kinetic coupling between the ZZO and M-22 components in the tandem catalytic system. To validate this hypothesis at the molecular level, density functional theory (DFT) calculations were conducted to compare three potential intermediates–CH2O*, CH3O*, and CH3OH in their ability to migrate from ZZO into sinusoidal channels of M-22. For the methanol-mediated pathway (Fig. 5a m1-m6), the mechanism initiates with methanol adsorption at the Brønsted acid site in the T8 position of sinusoidal channel in M-22 (Fig. 5a). Subsequent dehydration through transition state mTS2 generates a methyl group (CH3) and water, requiring a substantial activation barrier of 2.05 eV (Fig. 5b). Following water desorption (Fig. 5a m4), toluene enters the framework where the CH3 group attacks the para-carbon of the aromatic ring (Fig. 5a m5), ultimately yielding desorbed PX (Fig. 5a m6).

In contrast, the CH x O*–mediated pathway demonstrates superior thermodynamic feasibility (Fig. 5a f1-f11). Initial adsorption of CH2O* at the T8 Brønsted site (Fig. 5a f1) is followed by protonation to form a hydroxymethyl cation (CH2OH*) (Fig. 5c f2). Concurrent toluene ingress enables para-carbon attack via transition state fTS1 (Fig. 5c), forming H--HO-CH2-Tol* (Fig. 5c f3-f4). The rate-determining step involves C-O bond cleavage through transition state fTS2 (1.19 eV barrier) (Fig. 5c f5), yielding CH3-Tol* and water (Fig. 5c f6). Subsequent hydrogen transfer from surface H* species (Fig. 5c f8) via transition state fTS3 (Fig. 5c f9) generates CH3-Tol*, culminating in PX desorption (Fig. 5c f11). Notably, CH x O* exhibits substantially higher adsorption energies compared to both CH3O* (+1.0 eV) and CH3OH (+3.0 eV) (Fig. 5d), rendering it thermodynamically unfavorable for alkylation within M-22. The maximum activation barrier of CH x O*-mediated pathway (1.19 eV) remains significantly lower than that of methanol barrier (2.05 eV), conclusively identifying CH x O* as the kinetically favored intermediate.

In summary, this work demonstrates a tandem catalytic strategy for direct PX synthesis via CO2 hydrogenation coupled with toluene methylation using a ZZO & M-22 system. Compartmentalized catalysis enables synergy: ZZO preferentially activates CO2/H2, while M-22 channels mediate C–C coupling via CH x O* intermediates. This approach overcomes thermodynamic constraints of conventional toluene methylation, achieving 91% xylene selectivity (70.4% PX/xylene) at 10.3% toluene conversion. TEOS modification of M-22 is critical, passivating supercage Brønsted acid sites (Si(OH)Al) via TEOS deposition to suppress disproportionation side reactions. Mechanistically, CH x O* intermediates kinetically couple CO2 activation and alkylation, bypassing methanol dehydration. Controlled diffusion of CH x O* from ZZO to M-22 optimizes thermodynamic-kinetic balance, facilitating confined electrophilic para-methylation. This establishes a paradigm for tandem catalyst design through zeolite architecture engineering, where precise intermediate dimension/configurational control governs selectivity. The strategy offers an efficient CO2 valorization route and a carbon recycling alternative to conventional PX production, underscoring structure-directed catalysis in bridging CO2 utilization with aromatics synthesis.

Methods
Materials
Zn(NO3)2·6H2O were purchased Sinopharm (China), (NH4)2CO3, m-xylene (99%), toluene (99.9%) and Zr(NO3)4·5H2O, tetraethyl orthosilicate (TEOS, 99.98%) and n-hexane (99%) were obtained from Macklin, CO2/H2/Ar [24/72/4 (v/v/v)], H2 (99.99%) and Ar (99.99%) were obtained from Lanzhou Yulong Gas Co., Ltd. HMCM-22 (Si/Al = 15) were obtained from Nankai Catalysts.

Preparation of catalysts
The ZnZrO catalyst that doped 20% ZnO (Zn/(Zn+Zr) molar ratio) was prepared as follows. 14.9 g of Zn(NO3)2·6H2O and 8.6 g of Zr(NO3)4·5H2O were dissolved in a flask by using 400 mL of deionized water, then the 250 mL aqueous solution of (NH4)2CO3 (43.2 g) as precipitant was dropwise added into the aforementioned solution under vigorous stirring at 70°C to form a precipitate. The obtained suspension was continuously stirred for 4 h at 70°C, followed by cooling down to room temperature and filtering. The precipitate was washed three times with deionized water, dried at 110°C for 4 h, and calcined at 500°C in static air for 3 h.

Si/M-22 zeolite was prepared by tetraethoxysilane (TEOS) modification. Typically, the steps to modify the M-22 once were to immerse 1 g M-22 into the mix solution of 1 g TEOS and 20 ml of n-hexane for 4 h; after that, organic solvent was removed by evaporation at 80°C and the powder was dried overnight at 110°C, followed by calcination at 550°C for 4 h in air. While the above steps were repeated, a new symbol of nSi/M-22 (n means the number of repetitions) was used. After repeating the procedure above for four times, the 4Si/M-22 was obtained. The tandem catalyst was typically prepared by physical mixing. The weight ratio of oxides and zeolites was 1:1. For preparation of tandem catalyst ZZO & M-22, ZZO and M-22 were grinded in an agate mortar for 5 min, pressed under 10 MPa and granulated into the required size in the range of 40-80 mesh.

M-22 with the pre-coked supercages, denoted as pre-coked M-22 was prepared following the procedure: M-22 was pretreated at 500°C for 2 h and then cooled down to 350°C under Ar flow (100 mL min−1). Subsequently, the transformation of m-xylene (99%) was carried out at 350°C, atmospheric pressure and 8 h−1 of m-xylene WHSV with Ar flow (40 mL min−1) for 12 h, to form pre-coked M-22.

Catalyst evaluation
Catalytic performance was evaluated on a high-pressure fixed-bed reactor equipped with a quartz sample tube. Typically, 0.52 g catalyst (40-80 mesh) were loaded into the quartz sample tube without mixed quartz sand. Before evaluating the catalytical performances, raised to the specified temperature under the protection of argon. For the investigation of toluene methylated with CO2 and H2, a mixture gas of CO2/H2/Ar [24/72/4 (v/v/v)] was induced into a quartz sample tube, and then toluene was carried into reaction tube (90°C) at a constant flow rate by using a piston pump. The reaction was conducted at 320°C and 3.0 MPa, with a GHSV of 12,000 mL g−1 h−1 and a toluene weight hourly space velocity (WHSV) of 1 h−1, and the data were collected at the reaction time of 12 h. For the contrast experiment of toluene methylated with methanol/Formaldehyde (s-Trioxane), a mixed solution of C7H8/CH3OH or HCHO [5/1 (mol/mol)] was pumped into reaction tube with a toluene WHSV of 1 h−1 at 320°C / 260 320°C and 3.0 MPa, while 12,000 mL g−1 h−1, H2/Ar was used as a carry gas simultaneously. In the quantitative analysis, we used iphenyl as an internal standard to determine the C balance, and the C balance was above 96%. The liquid products were analyzed by using the gas chromatography equipped with CBX-WAX columns to analyze aromatics, the conversion of toluene and production selectivity was calculated as follows:

Conv: (%) = n i / n in toulene × 100 (1)

Sel (%) = P i / n i × 100 (2)

Where n i is the molar amount at the outlet of the individual aromatic product i.

Catalyst characterization
The XRD analysis was performed using a Philips PW1050/81 diffractometer equipped with a Cu Kα radiation source (λ = 1.5418 Å). The instrument was operated in Bragg-Brentano geometry at 40 kV and 30 mA.

The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method. The textural properties of the samples, including specific surface area and pore size distribution, were characterized by N2 physisorption using a Microtrac BEL BELSORP-mini II analyzer. Prior to analysis, approximately 0.2 g of each catalyst was subjected to degassing at 300°C for 2 h.

The microstructural and elemental composition of the catalysts were characterized by aberration-corrected HAADF-STEM and STEM-EDX mapping, respectively, using an FEI Cubed Titan G2 60-300 microscope at an accelerating voltage of 300 kV. Morphological analysis was performed by scanning electron microscopy (SEM) on a Thermo Fisher Scientific Apreo S instrument. The SEM specifications were as follows: electron gun: field emission (FEG); accelerating voltage: 30–200 kV.

CO2 and NH3 temperature-programmed desorption (TPD) experiments were conducted using a Micromeritics ASAP 2920 system. Approximately 0.1 g of catalyst, loaded in a quartz reactor, was first pretreated at 500°C under Ar flow for 3 h and subsequently cooled to 50°C. The sample was then saturated with a flow of either 10 vol% CO2/He or 10 vol% NH3/He (50 mL min−1) for 1 h at this temperature. Subsequently, the flow was switched to pure He for 1 h at 50°C to purge physically adsorbed molecules. After stabilizing the baseline, the temperature was ramped to 700°C at a rate of 10°C min−1 under He flow, and the desorption signal was monitored by a thermal conductivity detector (TCD).

In-situ DRIFT spectra of surface species formed from the CO2/13CO2 + H2/D2 reaction. Prior to the experiment, the samples were heated to 450°C for 2 h under an Ar atmosphere to remove any water contained in the cell and on the catalyst surface. In situ DRIFT spectra of surface species over ZZO from CO2 + H2 and subsequently switched to D2. Reaction conditions: ZZO, 0.1 MPa, 320°C, 30 mL min−1 CO2 + H2, D2 10 mL min−1. In situ DRIFT spectra of surface species over ZZO & 4Si/M-22, 30 ml/min CO2 + 10 mL min−1 H2 + toluene, subsequently switched to 13CO2 was injected into the cell, and infrared spectrum information of the catalyst surface and species was collected.

FTIR spectra of pyridine was performed for both M-22 and nSi/M-22. The sample (about 30 mg) was pressed into a wafer, and then treated under vacuum at 400°C for 1 h. After cooling to 50°C, pyridine was introduced to the transmission cell until saturation. Then, the signal vs. the reference signal was recorded at 150°C until the signal unchanged.

Magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectra of 1H and 27Al were collected on a Bruker Avance III 600 MHz Wide Bore spectrometer operating at a magnetic field of 14.1 T, 4 mm dural-channel probe; the rotate speed of samples is 10.0 kHz; The chemical shifts for 27Al MAS NMR spectra were referenced to the aqueous solution of Al(NO3)3 (0 ppm), respectively. The chemical shifts for 1H MAS NMR spectra were referenced to tetramethylsilane (TMS). Quantitative 1H MAS NMR measurement was performed by comparing the signal area of the zeolite sample under study with that of adamantine as an external standard.

Quasi in situ 13C CP MAS NMR experiment and Quasi in situ 2D 13C-1H HETCOR MAS NMR experiment are carried out as follows: the basic experimental device is a home-built high-pressure fixed-bed catalytic reactor. Standard reaction conditions: 3 MPa, 320°C, 10 mL min−1 H2 + 60 mL min−1 13CO2. The reactor was quickly immersed into liquid nitrogen to quench the reaction, so that highly reactive intermediates on the catalysts can be capture. Subsequently, the reactor was sealed, after waiting for recovery to room temperature, and further transferred into 4 mm rotor for further NMR experiment.

In situ SVUV-PIMS experiments were carried out beamline BL03U at the National Synchrotron Radiation Laboratory, Hefei, P.R. China. This technique is employed for the detection of oxygen-containing intermediates during the CO2 hydrogenation process. The photoionization efficiency spectra were collected for 300 s in the energy range of 10.5-11.3 eV with a step of 0.05 eV. Reaction conditions: the ZnZrO catalyst (0.5 g); reaction gas (H2/CO2 = 3/1), at 300°C, 0.8 MPa, GHSV = 12,000 mL gcat −1 h−1.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this study, Li et al. developed a tandem catalyst system consisting of ZnZrOx and HMCM-22, which enables the highly selective production of high-value para-xylene via the CO2 hydrogenation coupled toluene methylation reaction. The same reaction and similar catalyst components were studied such as those in Ref. 17 and 18. Precise modulation of the (new) HMCM-22 structure effectively suppressed the toluene disproportionation side reaction, resulting in exceptional PX selectivity, while simultaneously demonstrating that formaldehyde serves as the key alkylation intermediate.\n1. Does the supercage of MCM-22 contribute to the formation of PX?\n2. When MCM-22 is modified with TEOS, does its structure undergo any changes, and how does the consequent alteration in acidity affect the formation of PX?\n3. Does the metal oxide ZnZrOx also play a role in activating toluene in this reaction?\n4. In Supplementary Fig. 7a, the control experiments performed by the authors show that using formaldehyde as the alkylating agent leads to a product distribution similar to that obtained with CO2 and H2, whereas the use of methanol results in a markedly different outcome. What do the authors propose as the fundamental reason for this discrepancy? Could it be that the zeolite exhibits selectivity toward different CHxO intermediate species?", "reviewer_2": "This work reported a tandem catalytic strategy for direct PX synthesis via CO2 hydrogenation coupled with toluene methylation. The ZZO&HMCM-22 system can achieve a 91% xylene selectivity (70.4% PX/xylene) at 10.3% toluene conversion. TEOS modification of HMCM-22 is very routine, but some interesting and important results were reported and discussed, including the use of HMCM-22 (rather than HZSM-5) for PX synthesis, with formaldehyde identified as the key oxygenated intermediate transferred to the zeolite. Moreover, the manuscript is well written. However, there are some concerns that the authors should address well before further considering publication. The detailed comments are as follows:\n1. More relevant references in this field in the introduction section should be added to ensure a more comprehensive representation of the current state of research.\n2. In the formula of the conversion of toluene and production selectivity, ni is the molar amount at the outlet of the individual product i. I suggest ni is the molar amount at the outlet of the individual aromatic product i. This clarification ensures corresponding to Figures, excluding non-aromatic byproducts.\n3. The specific surface area and pore volume of both ZZO and HMCM-22 materials should be given to elucidate their influence in catalytic performance.\n4. The authors noted that HZSM-5-based systems deliver moderate xylene yields. Therefore, I suggest the author also show the comparison of xylene yields with other catalytic systems reported in the literature. A literature comparison with such catalytic systems would strengthen the evaluation of their performance.\n5. HMCM-22 zeolite possesses two-dimensional sinusoidal 10-MR channels, and supercage configurations composed of 12-MR. However, the two pore structures are not interconnected. How does the catalytic lifetime of ZZO/M-22 compared to that of the conventional ZZO/ZSM-5?\n6. The inclusion of CO2 conversion and CO selectivity is advised to provide a more comprehensive assessment of the tandem system's capabilities.\n7. Formaldehyde species generated on ZnZrOx and their subsequent transfer to HMCM-22 is less reported in previous studies. What are the distinct characteristics of this tandem syntem when compared to conventional methanol-mediated pathways?\n8. After TEOS modification, the change in the surface acidity of HMCM-22 should be given via 2,6-DTBPy-IR analysis or 1, 3, 5-triisopropylbenzene cracking.\n9. Fig. 1b shows no aromatics were detected in CO2 hydrogenation over ZZO&M-22. What are the predominant products? Is there effect of these products on toluene methylation?"}, "author_response": "Response to the Reviewers’ Comments\nAccording to reviewers’ suggestions, the point-by-point responses are listed below.\nReviewer #1 (Remarks to the Author):\nIn this study, Li et al. developed a tandem catalyst system consisting of ZnZrOx and\nHMCM-22, which enables the highly selective production of high-value para-xylene\nvia the CO hydrogenation coupled toluene methylation reaction. The same reaction\n2\nand similar catalyst components were studied such as those in Ref. 17 and 18. Precise\nmodulation of the (new) HMCM-22 structure effectively suppressed the toluene\ndisproportionation side reaction, resulting in exceptional PX selectivity, while\nsimultaneously demonstrating that formaldehyde serves as the key alkylation\nintermediate.\n1. Does the supercage of MCM-22 contribute to the formation of PX?\nResponse: Thank you for your valuable comments. The supercages is unfavourable to\nthe selective formation of PX, but rather promotes the disproportionation of toluene.\nAccording to literature reports [1], the acidity within the supercages of MCM-22\ncan be selectively poisoned by coke deposition from m-xylene. Therefore, we treated\nMCM-22 with m-xylene to induce such coking, the results are shown in Figure R1. As\nobserved, after the supercages were deactivated by coking, the toluene conversion\ndecreased from 15% to 13%, the selectivity towards benzene dropped significantly\nfrom 11% to zero. The 2% reduction in the toluene conversion is consistent with the\nconversion rate of toluene disproportionation on the sole MCM-22. These results\nindicate that the presence of the supercages is not conducive to the selective formation\nof PX, but rather promotes the disproportionation of toluene.\nFigure R1. The catalytic performance of ZZO&M-22 and ZZO&M-22pc (MCM-22\nwith the pre-coked supercages); Standard reaction conditions: 3 MPa, 320 oC, GHSV\nof a mixture gas (CO /H /Ar = 24:72:4) = 12000 mL g−1 h−1, and WHSV of toluene =\n2 2\n1 h−1.\n2. When MCM-22 is modified with TEOS, does its structure undergo any changes, and\nhow does the consequent alteration in acidity affect the formation of PX?\nResponse: Thank you for your valuable comments. After TEOS treatment, the\nmorphology of the M-22 remains unchanged via SEM (Figure R2) characterization.\nThe specific BET surface area, micropore area, external area and pore volume of M-22\nall decreased, while the pore diameter remained almost unchanged (Table R1; included\nas Table S2 in Supporting Information). The indicates that TEOS does not deposit\nextensively within the internal pores of the M-22, but predominantly at the entrances\nof all channels and on the external surface, thereby enhancing the shape selectivity of\nM-22. The description of this characterization has been added at the corresponding\nposition in the article.\nThese observations suggest TEOS not only passivates external surface pocket acid\nsites but also modulates supercages acidity, which is crucial for the high-selectivity\nformation of PX via using comprehensive acid characterization: NH -TPD, Py-FTIR,\n3\n1H MAS NMR, and 27Al MAS NMR. The acid sites on the external surface of MCM-\n22 facilitate xylene isomerization and deep-methylation reaction, while those located\ninside the supercages are primarily responsible for toluene disproportionation. The\npresence of these distinct acid sites collectively leads to a reduction in PX selectivity.\nFigure R2. a. and b. SEM images of 4Si/M-22.\nTable R1 The BET characterization results of the ZZO and MCM-22 catalysts\nBET t-Plot pore\nt-Plot Micropore t-Plot external\nCatalyst surface area volume diameter\nArea (m2/g) area (m2/g)\n(m2/g) (cm3/g) (nm)\nZZO 45 - - 0.06 -\nM-22 468 292 176 0.14 1.81\n1Si/M-22 425 299 126 0.15 1.85\n2Si/M-22 331 244 87 0.12 1.86\n3Si/M-22 329 243 86 0.12 1.87\n4Si/M-22 272 199 73 0.10 1.90\n5Si/M-22 256 185 71 0.09 1.87\n3. Does the metal oxide ZnZrO also play a role in activating toluene in this reaction?\nx\nResponse: Thank you for your valuable comments. The ZZO lacks the ability to activate\ntoluene. To confirm whether ZZO can activate toluene, we tested it as the sole catalyst.\nThe results show no formation of aromatic products in Figure R3, demonstrating that\nZZO lacks the ability to activate toluene.\nFigure R3. Catalytic Performance of CO + H toluene alkylation on ZZO. Standard\n2 2\nreaction conditions: 3 MPa, 320 oC, GHSV of a mixture gas (CO /H /Ar = 24:72:4) =\n2 2\n12000 mL g−1 h−1, and WHSV of toluene = 1 h−1.\n4. In Supplementary Fig. 7a, the control experiments performed by the authors show\nthat using formaldehyde as the alkylating agent leads to a product distribution similar\nto that obtained with CO and H , whereas the use of methanol results in a markedly\n2 2\ndifferent outcome. What do the authors propose as the fundamental reason for this\ndiscrepancy? Could it be that the zeolite exhibits selectivity toward different CH O\nx\nintermediate species?\nResponse: Thank you for your valuable comments. We attribute this observed\ndifference to the distinct alkylation intermediates involved. Crucially, when\nformaldehyde (CH O) was used as a methylating agent, replacing CO /H , side\n2 2 2\nproducts were formed. This result is consistent with that obtained when using CO + H₂\n2\nas the alkylation reagent. This observation suggests that CH O species, generated from\n2\nintermediates. These intermediates then undergo further reactions with toluene to\nproduce the observed identical side products. In contrast, methanol, as a methylating\nagent, did not lead to the formation of these compounds. Furthermore, formaldehyde\nexhibited more favorable reaction kinetics compared to methanol, facilitating PX\nformation at lower temperatures and achieving higher PX selectivity in Supplementary\nFig. 7b.\nHowever, regarding whether the zeolite exhibits selectivity for different CH O\nx\nintermediate species, we posit that there is selectivity of zeolite for different CH O\nx\nintermediate species. The essence of shape-selective catalysis lies in manipulating the\ninterplay between the structure of zeolites (catalyst) and the dimension/configuration\nof reactants/products/intermediates to controlthe selectivity towards desired products\nor the reaction pathways in a catalytic reaction. Our experimental results indicate that\nthe present zeolite system exhibits a preferential selectivity towards formaldehyde-like\nintermediates over other CHₓO species.\nReviewer #2 (Remarks to the Author):\nThis work reported a tandem catalytic strategy for direct PX synthesis via CO2\nhydrogenation coupled with toluene methylation. The ZZO&HMCM-22 system can\nachieve a 91% xylene selectivity (70.4% PX/xylene) at 10.3% toluene\nconversion. TEOS modification of HMCM-22 is very routine, but some interesting and important\nresults were reported and discussed, including the use of HMCM-22 (rather than\nHZSM-5) for PX synthesis, with formaldehyde identified as the key oxygenated\nintermediate transferred to the zeolite. Moreover, the manuscript is well written.\nHowever, there are some concerns that the authors should address well before further\nconsidering publication. The detailed comments are as follows:\n1. More relevant references in this field in the introduction section should be added to\nensure a more comprehensive representation of the current state of research.\nResponse: Thank you for your valuable comments. I have added more relevant\nreferences in this field in the introduction section. The specific references are as follows:\n17. Miao D, et al. Selective synthesis of para-xylene and light olefins from CO /H in\n2 2\nthe presence of toluene. Catal. Sci. Technol. 11, 4521-4528 (2021).\n18. Shang X, et al. Regulating anisotropic diffusion in zeolite for reinforced para-xylene\nsynthesis via CO hydrogenation in the presence of toluene. Appl. Catal. B Environ.\n19. Hong B, et al. Low-pressure CO hydrogenation coupled with toluene methylation\n2\nto para-xylene using atomic Pd-doped ZnZrO -HZSM-5. Appl. Catal. B Environ.\nx\nEnergy 361, 124606 (2025).\n20. Tuo J, et al. Shape-selective synthesis of para-xylene through tandem CO\n2\nhydrogenation and toluene methylation over ZnCeZrO /MCM-22 catalyst. Chin. J.\nx\nCatal. 73, 174-185 (2025).\n2. In the formula of the conversion of toluene and production selectivity, ni is the molar\namount at the outlet of the individual product i. I suggest ni is the molar amount at the\noutlet of the individual aromatic product i. This clarification ensures corresponding to\nFigures, excluding non-aromatic byproducts.\nResponse: According to the reviewer's suggestions, n has been revised in the formula\ni\nsection to represent the molar amount at the outlet of the individual aromatic product .\ni\n3. The specific surface area and pore volume of both ZZO and HMCM-22 materials\nshould be given to elucidate their influence in catalytic performance.\nResponse: We supplemented the BET experiment, and the results are presented in Table\nR2. (Table R2; included as Table S2 in Supporting Information). At the same time, the\ndescription of this characterization has been added at the corresponding position in the\narticle, as follows:\nAfter TEOS modification, the specific BET surface area, micropore area, external\narea and pore volume of M-22 all decreased, while the pore diameter remained almost\nunchanged. (Table R2). This indicates that TEOS does not deposit extensively within\nthe internal pores of the M-22, but predominantly at the entrances of all channels and\non the external surface, thereby enhancing the shape selectivity of M-22.\nTable R2 The BET characterization results of the ZZO and MCM-22 catalysts\nBET t-Plot pore\nt-Plot Micropore t-Plot external\nCatalyst surface area volume diameter\nArea (m2/g) area (m2/g)\n(m2/g) (cm3/g) (nm)\nZZO 45 - - 0.06 -\nM-22 468 292 176 0.14 1.81\n1Si/M-22 425 299 126 0.15 1.85\n2Si/M-22 331 244 87 0.12 1.86\n3Si/M-22 329 243 86 0.12 1.87\n4Si/M-22 272 199 73 0.10 1.90\n5Si/M-22 256 185 71 0.09 1.87\n4. The authors noted that HZSM-5-based systems deliver moderate xylene yields.\nTherefore, I suggest the author also show the comparison of xylene yields with other\ncatalytic systems reported in the literature. A literature comparison with such catalytic\nsystems would strengthen the evaluation of their performance.\nResponse: Thank you for your valuable suggestion. Based on the reviewers'\nsuggestions, we presented the performance in comparison with other literature in Table\nR3. (Table R3; included as Table S1 in Supporting Information). Compared to other\ncatalyst systems, this work (ZZO&4Si/M-22) can obtain a relatively high xylene\nselectivity of 91% while achieving a high PX of 70.4% selectivity, and can completely\ninhibit the occurrence of toluene disproportionation side reactions (Table R3)\nTable R3. A literature comparison with such catalytic systems\nBenzene\nToluene Xylene PX in\nCatalyst Ref.\nconversion selectivity xylene\nselectivity\nZnCrO -3P/Z5-Si3 10.6% ~50% 82.8% - (17)\nx\nZnZrOx&C-Z5–2.0–3Si ~19% ~91% ~90% ~1-2% (18)\n0.1PdZZO–3SiZ5(10) ~5% ~90 ~72% - (19)\nZnCeZrOx/MS13-2 23.6 92.4 67 ~1-2% (20)\nZZO-4SZ5 10% ~82% 70.8% ~1-2% (21)\nZrCuO -Z5 (25) ~22% - 93.8% ~3-7% (22)\n0.1 0.5\nZZO/Z5-SG 24.7%. 86.5% 25% - (42)\nZZO&4SiM-22 10.3 91% 70.4 No This work\n5. HMCM-22 zeolite possesses two-dimensional sinusoidal 10-MR channels, and\nsupercage configurations composed of 12-MR. However, the two pore structures are\nnot interconnected. How does the catalytic lifetime of ZZO/M-22 compared to that of\nthe conventional ZZO/ZSM-5?\nResponse: Thank you for your valuable comments. Compared with MCM-22 (2D,\nnon-interconnected pores), ZSM-5 has a 3D interconnected channel system with\nintersection cavities ~0.9 nm. We tested H-ZSM-5 (Si/Al ≈13.5, to match MCM-22) in\nthe conventional ZZO/ZSM-5 tandem. As shown in Figure R4, toluene conversion on\nZZO/ZSM-5 dropped from 15% to 6% over 120 h; xylene selectivity stayed ≈70%\nwhile PX selectivity in xylene rose slightly from ~25% to ~30%. Overall, ZZO/ZSM-5\nexhibited poorer stability than ZZO/M-22. We attribute the faster deactivation to larger\nintersection cavities of ZSM-5 and accessible strong acid sites, which promote\nformation of coke precursors (e.g., large polycyclic aromatics) and accelerate catalyst\ndeactivation.\nFigure R4. The stability of the conventional ZZO&ZSM-5; Standard reaction\nconditions: 3 MPa, 320 oC, GHSV of a mixture gas (CO /H /Ar = 24:72:4) = 12000 mL\n2 2\ng−1 h−1, and WHSV of toluene = 1 h−1.\n6. The inclusion of CO conversion and CO selectivity is advised to provide a more\n2\ncomprehensive assessment of the tandem system's capabilities.\nResponse: Thank you for your suggestion. According to your suggestions, we provided\nthe CO conversion and the CO selectivity for the ZZO&M-22 at different temperatures\n2\nand the optimal catalyst ZZO&4Si/M-22 at 320 oC, respectively. The results are\npresented in Figure R5. (Figure R5; included as Supplementary Fig 1 in Supporting\nInformation).\nAs can be seen from Figure R5a, for the ZZO catalyst, as the temperature increased\nfrom 280-400 oC, the CO conversion rose from 1.1% to 19.9%, and the selectivity of\nZZO&4Si/M-22 achieved a CO conversion of 4.2% with a CO selectivity of 22% at\n2\n320 oC.\nFigure R5. Catalytic Performance in CO Hydrogenation. (a) The effect of reaction\n2\ntemperature on ZZO&M-22 catalyst. (b) The catalytic performance of the optimal\ncatalyst ZZO&4SiM-22; Standard reaction conditions: 3 MPa, 320 oC, GHSV of a\nmixture gas (CO /H /Ar = 24:72:4) = 12000 mL g−1 h−1, and WHSV of toluene = 1 h−1.\n2 2\n7. Formaldehyde species generated on ZnZrO and their subsequent transfer to HMCM-\nx\n22 is less reported in previous studies. What are the distinct characteristics of this\ntandem system when compared to conventional methanol-mediated pathways?\nResponse: Thank you for your valuable comments. Compared with the conventional\nmethanol-mediated pathway, this tandem system shows several distinct features: (1)\ngreatly improved product selectivity — total xylene selectivity reaches 91% with para-\nxylene (PX) selectivity of 70.4%; (2) complete suppression of toluene\ndisproportionation; and (3) detection of side products derived from formaldehyde\nbeyond alkylation products (for example, 2-(m-methylbenzyl)toluene), which provides\ndirect evidence for a formaldehyde-mediated reaction pathway.\n8. After TEOS modification, the change in the surface acidity of HMCM-22 should be\ngiven via 2,6-DTBPy-IR analysis or 1, 3, 5-triisopropylbenzene cracking.\nResponse: According to the reviewers' suggestions, 2,6-di-tert-butyl-pyridine (2,6-\nDTBPy-IR) was carried out to discriminate the external Brønsted acid sites on the\nunmodified MCM-22 and 4Si/MCM-22 samples. The results show that TEOS\nmodification effectively poisons the external Brønsted acidity.\nAs shown in Figure R6, the characteristic bands at 1531, 1612, and 3365 cm−1 are\nattributed to the DTBPy adsorbed on the external surface Brønsted acid sites of MCM-\n22 zeolite, while the negative band at 3745 cm−1 corresponds to terminal silanol\nhydroxyl groups [2]. Compared with the unmodified MCM-22, the 4Si/MCM-22 sample\nexhibits significantly decreased for the intensity of the IR bands associated with both\nexternal surface Brønsted acid sites and terminal silanol groups. This indicates that\nTEOS modification effectively poisons the external Brønsted acidity.\nFigure R6. 2,6-DTBPy characterization of HMCM-22 and 4Si/HMCM-22. DTBPy was\nadsorbed onto the sample for 15 min at 150 °C, and evacuated for 1 h before the\nspectrum was recorded.\n9. Fig. 1b shows no aromatics were detected in CO hydrogenation over ZZO&M-22.\nmethylation?\nResponse: Thank you for your valuable comments. As shown in Figure R7, the\npredominant products from CO hydrogenation over ZZO&M-22 are methanol and\n2\ndimethyl ether, with only trace amounts of C –C olefins. When CO hydrogenation is\n2 4 2\ncoupled with toluene methylation, the amounts of methanol and dimethyl ether decrease\nmarkedly, and only trace C –C olefins are present; no aromatics from olefin alkylation\n2 4\nwith toluene are observed. These observations indicate that formaldehyde is the key\nintermediate reacting with toluene to form xylenes. Consumption of formaldehyde by\nthe methylation pathway therefore suppresses formation of methanol and dimethyl ether.\nFigure R7. The catalytic performance of CO hydrogenation over ZZO&M-22;\n2 2\n24:72:4) = 12000 mL g−1 h−1.\nReference\n[1]. Laforge, S. et al. m-Xylene transformation over H-MCM-22 zeolite 1.:\nMechanisms and location of the reactions. J. Catal. 220, 92-103 (2003).\n[2]. Ungureanu A, et al. An investigation of the acid properties of UL-ZSM-5 by FTIR\nof adsorbed 2, 6-ditertbutylpyridine and aromatic transalkylation test reaction. Appl.\nCatal. A Gen. 294, 92-105 (2005)."}, {"round": 2, "reviewer_comments": {"reviewer_1": "I read the revised manuscript and the Answer. The authors answered my questions very well and corresponding actions are included in the revised manuscript. The manuscript is acceptable with its present status.", "reviewer_2": "I appreciate the efforts made to revise the manuscript. Technically I do not object the publication of the revised manuscript, but I still do not consider the novelty of the reported results are high. In addition, other comments are as follows:\n1.Given that similar work (Chinese Journal of Catalysis 2025, 73, 174–185) has already been published, I suggest the author further emphasize the innovation of this work.\n2.Due to the additional addition of benzene, I do not believe this is a sustainable route for aromatic hydrocarbon synthesis. Therefore, I suggest modifying the relevant wording."}, "author_response": "Reviewer #2 (Remarks to the Author):\nI appreciate the efforts made to revise the manuscript. Technically I do not object the\npublication of the revised manuscript, but I still do not consider the novelty of the\nreported results are high. In addition, other comments are as follows:\n1.Given that similar work (Chinese Journal of Catalysis 2025, 73, 174–185) has already\nbeen published, I suggest the author further emphasize the innovation of this work.\nResponse: Thank you very much for your insightful comments. We have thoroughly\nexamined the recently published work (Chinese Journal of Catalysis 2025, 73, 174–185)\nby Tuo et al., which introduces a ZnCeZrO /MCM-22 catalytic system capable of\nx\nmethylating toluene through CO hydrogenation. This system achieved 92.4% xylene\n2\nselectivity, with 67% para-xylene (PX) selectivity. The authors suggested that methoxy\nor methanol intermediates could migrate from ZnCeZrO to MCM-22 zeolite\nx\nfacilitating the toluene methylation process. In our study, we designed a bifunctional\nZnZrO &HMCM-22 tandem catalyst that achieves 91% xylene selectivity, with 70.4%\nx\nbeing para-xylene. Most notably, we demonstrate for the first time that a formaldehyde-\nlike species (CH O*), rather than methoxy or methanol, serves as the crucial\n2\nintermediate for toluene methylation. We also verified the migration behavior of CH O*\n2\nfrom ZnZrO to HMCM-22. Our work not only advances the synthesis of PX through\nx\ntoluene methylation using CO hydrogenation but also uniquely illustrates how the\n2 2\nmethylation, as well as its migration behavior. This study highlights new formaldehyde-\nlike intermediates in CO hydrogenation, potentially paving new pathways for CO\n2 2\nutilization, including reactions where formaldehyde acts as a reagent.\n2.Due to the additional addition of benzene, I do not believe this is a sustainable route\nfor aromatic hydrocarbon synthesis. Therefore, I suggest modifying the relevant\nwording.\nResponse: Thank you for your valuable suggestion. we have revised the word\n\"sustainable\" to \" carbon recycling\"."}]

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