Targeted Synthesis of Durene from CO₂ via a Tailored Coupling Reaction Pathway

Abstract

A precisely engineered coupling reaction pathway was developed over a rationally designed multifunctional oxide-zeolite catalyst for the highly selective transformation of CO₂ to durene (1,2,4,5-tetra­methyl­benzene, termed 1,2,4,5-TeMB), a high-value-added aromatic compound. The detailed reaction pathway and dynamic structure evolution of the catalyst were unveiled through in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), resonant Auger electron spectroscopy (RAS), in situ X-ray absorption fine structure spectroscopy (XAFS), and density functional theory (DFT) calculations. An optimal oxide catalyst featuring the tetragonal InZrO _x solid solution structure was developed to match the thermodynamic characteristics of the CO₂-to-aromatics reaction. It exhibits a spatial synergy between CO₂ adsorption and H₂ activation, therefore promoting CO₂ hydrogenation via the HCOO* pathway at lower temperatures, which is thermodynamically essential for the formation of TeMBs over zeolites. For the selectivity control among several TeMB isomers, a ZSM-5 nanosheet zeolite with a b-axis dimension of subhundred nanometers was optimized. The presence of abundant surface acid sites and pore mouth confinement in these ZSM-5 nanosheets reinforces the pore mouth shape-selective methylation and precisely positions methyl substituents on benzene rings. Through efficiently coupling reaction processes of low-temperature CO₂ hydrogenation, aromatization, and zeolite pore mouth shape-selective methylation, the InZrO _x /ZSM-5 catalyst substantially narrows the C₆–C₁₂ aromatic distribution and achieves the targeted synthesis of durene, which accounts for 83.3% of the total aromatic products.

Introduction

The coupling of multifunctional catalysts provides a powerful strategy for constructing new reaction pathways with enhanced selectivity toward target products by integrating multiple elementary reaction processes within a single reactor. − Compared with simple reaction systems employing monofunctional catalysis, such coupled pathways inherently involve more complex but highly tunable reaction networks. For instance, the coupling reaction of CO₂ to aromatics is accomplished using a bifunctional catalyst that combines metal oxide and zeolite components (OXZEO), such as ZnAlO _x /ZSM-5, ZnCrO _x /ZSM-5, ZnZrO _x /ZSM-5, ZnZrO _x /ZSM-11, and In₂O₃–ZnZrO _x /ZSM-5. − However, the complex reaction network makes it difficult to selectively synthesize a single product. The resultant products of the CO₂-to-aromatics reaction constitute a mixture of C₆–C₁₂ aromatics, which typically display a quasi-normal distribution, with C₆–C₈, C₉, and C₁₀₊ comprising approximately 30%, 40%, and 30% of the total, respectively. , Although great efforts have been made to narrow the aromatic distribution, − the highly selective synthesis of a single high-value-added aromatic compound, such as 1,2,4,5-tetra­methyl­benzene (1,2,4,5-TeMB, durene), still remains a challenge. Durene functions as a key monomer for polyimides, which are in high demand owing to their application in photoresists, chip packaging, flexible displays, flexible printed circuits, and thermally conductive graphite films. The precise regulation of the reaction pathway, achieved through the rational design and integration of multifunctional catalytic components, provides a practical solution for the highly selective conversion of CO₂ to durene.

The CO₂-to-durene conversion in an oxide-zeolite coupling catalytic system follows a tandem reaction pathway that integrates the CO₂ hydrogenation, aromatization, and methylation processes. The precise thermodynamic and kinetic matching among these processes is crucial for achieving a highly selective synthesis of durene. Specifically, the production of higher polymethylbenzenes (polyMBs) over zeolite is more thermodynamically favorable at lower temperatures, whereas CO₂ hydrogenation over oxides is kinetically restricted. Therefore, it is necessary to develop oxide catalysts with an enhanced CO₂ hydrogenation activity at lower temperatures. Moreover, given that there are three TeMB isomers, the challenge lies in the specified location of methyl substituents for CO₂-to-durene conversion. The location of methyl substituents on benzene rings is governed by the steric constraints of zeolite micropores, which is a typical case of shape-selective catalysis. However, given the mismatch between the kinetic diameter of the bulky durene molecules and the micropore size of ZSM-5, the targeted production of durene cannot be effectively steered by the shape-selective catalysis of micropores alone.

As demonstrated in our previous studies, the straight channels and pore mouths of ZSM-5 along the b-axis function as the predominant spatial pathways for aromatization and methylation, respectively. , This work focuses on the combined engineering of spatial pathways within ZSM-5 zeolite and the low-temperature catalytic activity of oxide. Through the design of a multifunctional catalyst that integrates InZrO _x solid solution and ZSM-5 nanosheets, a precise reaction pathway was established and controlled toward the highly selective production of the key aromatic compound by coupling low-temperature CO₂ hydrogenation, aromatization, and pore mouth shape-selective methylation. The direct conversion of CO₂ results in an aromatic selectivity of 75.8% with a 94.2% proportion of TeMB among the total aromatics, a proportion substantially higher than the range of 66–83% reported in the literature. − Moreover, the targeted synthesis of durene is achieved, with durene accounting for up to 88.4% of all TeMB isomers.

Results and Discussion

Design of Coupled Catalyst for Highly Selective CO₂-to-Durene Conversion

In₂O₃-based oxides have been revealed as promising catalysts for the CO₂-to-methanol reaction under mild temperature conditions below 300 °C, but they often suffer from over-reduction into metallic In. , To overcome such limitations, a bimetallic oxide InZrO _x was screened as the oxide component in this study. A series of other well-reported oxides, namely ZnCrO _x , ZnZrO _x , and GaZrO _x , ,,, were also chosen for comparison with InZrO _x after being integrated with ZSM-5, and these bifunctional catalysts were denoted as M₁M₂-n/ZSM-5­(b). The detailed preparation procedures and labeling conventions are presented in the Experimental section of the Supporting Information. The CO₂ conversion of all these catalyst samples diminishes as the temperature decreases. Nevertheless, lower reaction temperatures are favorable for improving the selectivity to polyMBs. The notable low-temperature catalytic performance of InZrO _x /ZSM-5 is clearly demonstrated across a temperature range of 260–350 °C. For example, at a reaction temperature of 260 °C, InZr-4/ZSM-5(180) achieves a TeMB yield that is more than twice that of GaZr-8/ZSM-5(180) and about one-third higher than those of ZnCr-2/ZSM-5(180) and ZnZr-8/ZSM-5(180) samples (Table S1).

The catalytic performance of InZrO _x /ZSM-5 significantly depends on the content of In atoms. The In-rich InZr-2/ZSM-5(180) exhibits substantially lower aromatic selectivity across the entire temperature range of 220–350 °C, reaching a peak value of only 43.5% at 280 °C (Figure A). As the In content decreases, InZr-4/ZSM-5(180) displays the highest aromatic selectivity of around 82% within the comparatively lowest temperature range of 260–280 °C among the InZr-n/ZSM-5­(180) catalyst series. Upon a further reduction in the In content, the optimal temperature range of InZr-10/ZSM-5(180) shifts back to 300 °C, achieving a maximal aromatic selectivity of 79.7%, which is slightly lower than that of InZr-4/ZSM-5(180). For comparison, no catalytic activity is observed for ZrO₂ below 280 °C, and monometallic In₂O₃ primarily produces CH₄ and CO when assembled with ZSM-5(180) (Table S2). The apparent activation energies (E _a) of the series of InZr-n/ZSM-5­(180) catalyst samples were calculated to directly compare their catalytic performances (Table S2), and the lowest E _a value of 51.1 kJ mol^–1 was observed for the InZr-4/ZSM-5(180) sample. In addition to the effect of temperature, elevated pressure and reduced space velocity prolong the residence time of aromatic intermediates, thereby enhancing their successive methylation at the acidic sites of ZSM-5 and favoring the formation of TeMB (Figure S1). Under the reaction conditions of 280 °C, 8.0 MPa, H₂/CO₂ = 3:1, and 600 mL g^–1 h^–1, InZr-4/ZSM-5(180) exhibits a CO₂ conversion of 15.0% and an aromatic selectivity of 77.3%. Notably, the proportion of TeMB in the total aromatics reaches 86.0%, with durene accounting for 93.1% of the entire TeMB fraction (Figure B).

1.

Low-temperature catalytic performances of InZrO _x /ZSM-5 coupled catalysts and DRIFTS characterization of InZrO _x . (A) Organic product distribution over InZr-n/ZSM-5­(180) at various reaction temperatures. (B) CO₂ conversion, TeMB selectivity, and total aromatic selectivity of InZr-n/ZSM-5­(180) and ZrO₂/ZSM-5­(180) under the optimal reaction conditions. (C) A concise description of the DRIFTS characterization procedure. (D) DRIFTS spectra of InZr-n and ZrO₂ following the saturated adsorption of CO₂ and (E) after subsequent thorough exposure in H₂. The evolution of surface species in a H₂ atmosphere over (F) InZr-2 and (G) InZr-4 subsequent to the saturated adsorption of CO₂. Reaction conditions for (A): InZr-n/ZSM-5­(180) = 1:1, 3.0 MPa, H₂/CO₂ = 3:1, and 1200 mL g^–1 h^–1. Reaction conditions for (B): InZr-n/ZSM-5­(180) = 1:1 or ZrO₂/ZSM-5­(180) = 1:1, 280 °C, 8.0 MPa, H₂/CO₂ = 3:1, and 600 mL g^–1 h^–1.

The distinct catalytic performances across these InZr-n/ZSM-5 catalysts undoubtedly stem from the CO₂ hydrogenation process occurring over the InZr-n samples. Therefore, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was first employed to probe the CO₂ adsorption and hydrogenation processes on the InZr-n and ZrO₂ surfaces at ambient pressure. As shown in Figure C, the InZr-n and ZrO₂ samples were initially subjected to sequential treatment under H₂ and Ar atmospheres (step 1), followed by exposure to CO₂ for 30 min (step 2) and finally a switch to H₂ flow for 60 min (step 3). DRIFTS spectra were recorded during steps 2 and 3. Figure D,E presents a comparison of the DRIFTS spectra of different InZr-n and ZrO₂ samples after saturated CO₂ adsorption and subsequent H₂ treatment. The CO₂ adsorption species on the InZr-2 surface mainly consist of monodentate carbonate species (m-CO₃*), which are transformed into surface carboxyl groups (COOH*) through oxygen-end hydrogenation upon sufficient H₂ treatment. − On the InZr-4 surface, both monodentate and bidentate carbonate and bicarbonate species (m-CO₃*, b-CO₃*, HCO₃*, and b-HCO₃*) are observed after CO₂ adsorption, which are converted into formate (HCOO*) ,, and methoxy (CH₃O*) ,, species triggered by carbon-end hydrogenation. Detailed band assignments of these surface species on InZr-2 and InZr-4 are presented in Table S3. On In-deficient surfaces such as InZr-10 and ZrO₂, surface species were identified as similar to those on InZr-4 following CO₂ adsorption and H₂ treatment (Figure S2).

The temporal evolution of surface species on InZr-2 and InZr-4 during H₂ treatment is presented in Figure F,G. On the InZr-2 surface, the adsorbed carbonate species undergo a rapid decline upon exposure to H₂, concurrent with the emergence of COOH* species. These COOH* species are not readily amenable to further hydrogenation, as indicated by the nearly constant intensities of their characteristic peaks. On the InZr-4 surface, the carbonate and bicarbonate species experience a rapid decrease until they completely disappear. The absorption peak intensity of the HCOO* species initially increases upon exposure to H₂ and then gradually declines, whereas the characteristic peak of the CH₃O* species gradually intensifies with prolonged H₂ exposure.

DRIFTS spectra of the InZr-n and ZrO₂ samples under the simultaneous coadsorption of CO₂ and H₂ at various temperatures were also measured (Figure S3). The m-CO₃* and COOH* species were identified on the InZr-2 surface at a low temperature of 100 °C. With increasing temperature, the characteristic peak intensity of m-CO₃* gradually decreases, while the COOH* peak initially increases and subsequently decreases. For InZr-4, the surface species at 100 °C are primarily composed of bicarbonate species, accompanied by a small amount of m-CO₃* and HCOO* species. Upon elevating the temperature to 200 °C, the characteristic peak intensity of bicarbonate species decreases markedly, concurrent with an increase in the intensity of the HCOO* peak. With a further temperature increase, the intensity of the HCOO* peak gradually increases, reaching a maximum at around 280 °C, and then gradually declines. Furthermore, characteristic peaks of CH₃O* species were also detected at elevated temperatures.

Low-Temperature CO₂ Hydrogenation on t-InZrO _x Solid Solution

Having obtained the in situ information on surface species for CO₂ hydrogenation, it is essential to unveil the origin of the observed variations in surface speciation and its effect on catalytic behavior. We conducted a systematic characterization of the InZr-n oxides with a focus on their crystal structure, elemental distribution, and coordination environment. ICP-AES (inductively coupled plasma‑atomic emission spectroscopy) analysis indicates that the bulk elemental compositions of InZr-n samples are in line with their feeding ratios (Table S4). XRD (X‑ray diffraction) and Raman analyses demonstrate that, unlike the mixed phases of monoclinic (m-ZrO₂) and tetragonal (t-ZrO₂) in monometallic oxide ZrO₂, ,, all InZr-n samples are predominantly composed of a single tetragonal phase, ,, with no distinct characteristic peaks corresponding to the In₂O₃ phase being detected (Figures S4 and S5). This can be ascribed to the close lattice matching between cubic In₂O₃ (c-In₂O₃) and t-ZrO₂. To clarify the effect of the ZrO₂ phase, both m-ZrO₂ and t-ZrO₂ components were synthesized (Figure S4) and evaluated in the CO₂ hydrogenation reaction after integration with ZSM-5(180) (Table S5). Although t-ZrO₂/ZSM-5­(180) exhibits high aromatic selectivity, it suffers from low catalytic activity, which is significantly enhanced with the introduction of In atoms to construct the InZrO _x binary oxide. In contrast to monometallic ZrO₂, the characteristic diffraction peaks of the t-ZrO₂ phase in InZr-n samples gradually shift to higher angles as the In content increases (Figure S4), indicating the lattice contraction of t-ZrO₂. HRTEM-EDS (high‑resolution transmission electron microscopy coupled with energy dispersive X‑ray spectroscopy) analysis further confirms a high dispersion of In and Zr species in InZr-4, while local aggregation of the c-In₂O₃ phase is observed in InZr-2 (Figures A and S6). ,

2.

Structural characterizations of InZr-n samples. (A) HRTEM images of InZr-2 and InZr-4 and corresponding EDS elemental maps of indium and zirconium. (B) O RAS spectra of In₂O₃, ZrO₂, InZr-2, and InZr-4. (C) The evolution of the In–O and Zr–O coordination numbers for InZr-2 and InZr-4 during H₂ treatment, CO₂ adsorption, and CO₂ hydrogenation reaction, determined using in situ EXAFS.

XAFS (X-ray absorption fine structure) and XPS (X-ray photoelectron spectroscopy) were utilized to characterize the valence states and coordination environments of the InZr-n samples. The average valence states of In and Zr in all InZr-n samples are approximately +3 and +4, respectively (Figures S7 and S8), , while the coordination numbers of In–O and Zr–O are markedly lower than those in In₂O₃ and ZrO₂ (Figure S7, Tables S6 and S7). RAS (resonant Auger electron spectroscopy) allows for the discrimination between electronic transitions and energy transfer processes of subtle intensity variations. , The two-dimensional O RAS spectra of In₂O₃, ZrO₂, and InZr-n samples are illustrated in Figure B. Lattice oxygens in the form of In–O–In bonds (photon energy: 533.5 and 538.0 eV) and Zr–O–Zr (photon energy: 532.1, 535.5, and 541.7 eV) are detected from the O RAS spectra of In₂O₃ and ZrO₂, corresponding to the electron transition process from the O 1s orbital to the O 2p-In 5s/5p hybrid orbital and to the O 2p-Zr 4d/5s/5p hybrid orbital, respectively. The O RAS spectra of InZr-2 demonstrate the presence of both Zr–O–Zr and In–O–In configurations. In contrast, InZr-4 exhibits a distinct characteristic peak at 532.8 eV (photon energy), which is assigned to the electron transition from the O 1s orbital to the In 5s/5p-O 2p-Zr 4d/5s/5p hybrid orbital, indicating the dominant form of lattice oxygen in the In–O–Zr moiety. This result further verifies that In atoms are highly dispersed in the t-ZrO₂ matrix with minimal formation of the independent In₂O₃ phase.

The structural characteristics of InZr-n samples thus can be deduced from these characterization results. In ZrO₂ and In₂O₃, Zr and In atoms display octahedral and hexadentate coordination configurations, respectively, with Zr–O and In–O coordination numbers of 8 and 6. The respective values decrease to 6.9–7.6 and 4.5–5.6 in the InZr-n samples. Furthermore, an increase in the In content results in a steady decrease in the Zr–O coordination number and a gradual increase in the In–O coordination number. Notably, both Zr–O and In–O coordination numbers demonstrate a tendency to stabilize at around 7.0 and 4.7, respectively, as the composition of the InZrO _x sample approaches that of InZr-4 (Figure S9). Drawing upon Shannon’s effective ionic radii table, the ionic radius of In³⁺ (4.7) is approximately 83 pm, slightly smaller than that of Zr⁴⁺ (7.0), which is 92 pm. Consequently, a portion of Zr⁴⁺ sites in t-ZrO₂ can be substituted by In³⁺ in the InZr-n system, leading to lattice contraction and local charge imbalance and thereby promoting the formation of oxygen vacancies and other coordinatively unsaturated structures. Among all InZr-n samples, InZr-4 exhibits predominant In–O–Zr bonds and the highest degree of coordinative unsaturation for both In–O and Zr–O bonds. Consequently, it possesses abundant surface defect structures (Figure S8 and Table S4), along with minimal local aggregation of the In₂O₃ phase.

To clarify the intrinsic active sites for CO₂ adsorption and hydrogenation, in situ EXAFS and quasi in situ XPS were utilized to characterize the dynamic structure evolution of InZr-4 and InZr-2 (Figures S10 and S11). The in situ EXAFS results for InZr-4 indicate that after H₂ treatment, the In–O and Zr–O coordination numbers decrease from 4.7 and 7.0 to 4.2 and 6.2, respectively (Figure C, Tables S8 and S9). Subsequently, following CO₂ adsorption and the subsequent CO₂ hydrogenation reaction, the In–O and Zr–O coordination numbers substantially revert to 4.6 and 7.0, respectively. Quasi in situ XPS further corroborates the formation of oxygen vacancies after H₂ treatment and their consumption during the CO₂ adsorption and hydrogenation reaction (Figure S10). These findings demonstrate that H₂ treatment promotes the formation of coordinatively unsaturated In–O and Zr–O sites in InZr-4, which are involved in the CO₂ adsorption and hydrogenation reaction. In contrast to InZr-4, the decrease in the In–O coordination number of InZr-2 during H₂ treatment cannot be fully recovered during the subsequent CO₂ adsorption and hydrogenation reaction, which is ascribed to the irreversible reduction of the In₂O₃ phase (Figure S12). Indeed, the reduction of the In₂O₃ phase and migration of In during this process could potentially reduce the CO₂ hydrogenation activity and result in the undesirable excessive hydrogenation of intermediate species. In summary, the InZr-4 sample displays a tetragonal InZrO _x solid solution structure, with a lattice substitution ratio of up to 24% and minimal local aggregation of In₂O₃. This lattice substitution ratio not only facilitates the formation of coordinatively unsaturated In–O and Zr–O structures, which serve as the active sites for CO₂ adsorption and hydrogenation, but also stabilizes the valence state of In and inhibits its reduction. ,

We are now in the position to address the detailed reaction pathway of CO₂ hydrogenation to methanol by employing periodic density functional theory (DFT) calculations (Figure ). All possible permutations and combinations were enumerated for both bulk and surface structures of InZr-4 and InZr-2 samples according to the experimental In/Zr ratios (Tables S10–S13), from which the optimal locations of In atoms and oxygen vacancies within the bulk and on the surface of InZr-4 and InZr-2 samples were identified. An In–O–In chain structure is present on the InZr-2 surface, whereas only isolated In–O–In motifs are observed on the InZr-4 surface (Figures A and S13). This finding further confirms the aggregation tendency of In atoms at higher contents. On both surfaces, In–O sites display lower dissociation activation enthalpies of the H₂ molecule compared to Zr–O sites (Figure S14). On the contrary, CO₂ exhibits stronger adsorption on Zr–O sites than on In–O sites (Figure S15). It is noteworthy that the physisorption of CO₂ on surface oxygen vacancies is significantly weaker than the chemisorption on Zr–O sites in both structures. All these results align with the (quasi) in situ structural characterizations of InZr-n samples and more clearly illustrate that the activation of H₂ and the adsorption of CO₂ occur on surface In–O and Zr–O sites, respectively.

3.

Theoretical calculations. (A) Suggested bulk and surface structures of InZr-4 and InZr-2 for DFT calculations. Red: O atom; dark blue: In atom; light blue: Zr atom. (B) Gibbs free energy profiles of CO₂ hydrogenation to methanol via COOH* and HCOO* pathways on InZr-2 and InZr-4 surfaces at 280 °C.

Two possible reaction pathways (HCOO* vs COOH*) of CO₂ hydrogenation to methanol are summarized in Figure S16. The calculated Gibbs free energy profiles of CO₂ hydrogenation at 280 °C on the surfaces of InZr-4 and InZr-2 are depicted in Figure B, and the corresponding transition state structures are summarized in Figure S17 and S18. The coadsorption of CO₂ and H₂ is stronger on the InZr-2 surface than on the InZr-4 surface. The overall Gibbs free energy barriers are calculated to be 1.60 and 1.97 eV on the InZr-4 surface and 2.12 and 1.91 eV on the InZr-2 surface for the hydrogenation of CO₂ via HCOO* and COOH* pathways, respectively. Such results evidence that the HCOO* pathway predominates on InZr-4, whereas the COOH* pathway prevails on InZr-2. InZr-4 exhibits higher catalytic activity than InZr-2 (1.60 vs 1.91 eV). The dissociation of H₂COOH* to H₂CO* and the hydrogenation of COOH* are identified as the rate-determining steps on InZr-4 and InZr-2 surfaces, respectively (Figure B). As a result, HCOO* and COOH* are respectively identified as the key intermediates on the InZr-4 and InZr-2 surfaces, in line with the DRIFTS characterizations from the experiments (Figure ).

We further studied the catalytic activity of CO₂ hydrogenation on the InZr-10 structure, in which only one In atom replaces a Zr atom on the surface (Figure S13). Despite the preference for the same HCOO* pathway (Figures S19 and S20) and a similar composition of surface species as observed via DRIFTS on both the InZr-4 and InZr-10 surfaces, InZr-4 exhibits a much higher hydrogenation activity (Figure S21). This may be ascribed to the spatial synergy between In–O and Zr–O sites especially within the unsaturated In–O–Zr coordination structure, which cooperatively facilitates H₂ heterolytic dissociation and CO₂ chemisorption and delivers the highest aromatic STY among InZr-n samples after integrated with ZSM-5(180) (Figure S9). Furthermore, upon integration with ZSM-5, the InZr-4/ZSM-5(180) coupled catalyst exhibits improved CO₂ conversion efficiency and reduced apparent activation energy relative to bare InZr-4 under identical reaction conditions (Table S14). Comparison of the DRIFTS spectra during the coadsorption of H₂ and CO₂ reveals greater abundances of carbonate and bicarbonate intermediates on bare InZr-4 than on InZr-4/ZSM-5(180), particularly at low temperatures (Figure S22). Accordingly, the ZSM-5-induced “pull driving effect” , is mainly demonstrated by the accelerated consumption of surface carbonate/bicarbonate species.

Pore Mouth Shape-Selective Methylation to Durene on ZSM-5 Nanosheets

In addition to the “pull driving effect”, the more important role of ZSM-5 in the InZrO _x /ZSM-5 catalytic system is further manifested in the tandem reaction pathway involving aromatization and methylation. Through adjusting the Si/Al ratio of ZSM-5 within the range of 50–200 (Table S15, Figures S23 and S24), the aromatic selectivity varies and reaches 74–77% when the Si/Al ratio lies within the range of 100–150 (Table S16). Meanwhile, the methylation degree of aromatic rings exhibits a stronger correlation with the size of ZSM-5.

The ZSM-5 zeolite features two sets of 10-membered ring (10MR) channels along the a- and b-axes: sinusoidal channels along the a-axis (5.5 × 5.1 Å) and straight channels along the b-axis (5.6 × 5.3 Å). , In the InZr-4/ZSM-5­(b) coupled catalytic system, the parameter b denotes the b-axis thickness of the nanosheet ZSM-5 zeolite, which ranges from 80 to 350 nm. A lower b value corresponds to a higher proportion of TeMB in the total aromatics (Table S16, Figure A). By decoupling the aromatization and methylation reactions, the substitution ratio of aromatic rings (S _C9+/A) was defined, and it demonstrates a linear correlation with the reciprocal of the b-axis dimension of ZSM-5 (1/b) (Figure S25). , Owing to the anisotropic characteristics of the two sets of 10MR channels in ZSM-5, , aromatic molecules experience lower diffusion resistance along the b-axis straight channels. Hence, the formation and diffusion of aromatic species within the ZSM-5 framework are restricted along the b-axis, which acts as the dominant spatial pathway. The nanosheet morphology obtained by shortening the b-axis results in shorter straight channels with increased exposure, which enhances the methylation reactions at the pore mouths and external surfaces and improves the selectivity toward polyMBs. Upon reducing the b-axis thickness to be lower than 100 nm, the proportion of TeMB in the total aromatics reaches approximately 90% (Table S16).

4.

Regulation of the aromatic distribution over InZr-4/ZSM-5­(b). (A) CO₂ conversion, aromatic selectivity, and aromatic distribution over InZr-4/ZSM-5­(b) with a Si/Al ratio of 100 and various b-axis thicknesses. a: 80 nm; b: 100 nm; c: 180 nm; and d: 350 nm. Reaction conditions: InZr-4/ZSM-5­(b) = 1:1, 8.0 MPa, H₂/CO₂ = 3:1, and 600 mL g^–1 h^–1. (B) iDPC-STEM images of empty ZSM-5 straight channels and ZSM-5 straight channels after the adsorption of 1,2,4- or 1,3,5-TriMB, viewed projected along [010]. Scale bars, 2 nm. (C) Adsorption capacity curves of TriMB isomers at 0.2 kPa and 40 °C. (D) Initial diffusion fitting for characteristic diffusion behavior, calculated from experimental results between Q _t /Q _∞ = 0–0.25 and normalized based on the final equilibrium uptake values. ,

However, considering that there are three TeMB isomers, achieving a highly selective synthesis of durene necessitates the specified positioning of methyl substituents on the benzene ring. The CO₂ hydrogenation reaction results over the InZr-4/ZSM-5­(b) catalyst series demonstrate that the proportion of durene in TeMB ranges from 87% to 95%, far exceeding the thermodynamic equilibrium composition. , Furthermore, the durene proportion in TeMB rises with increasing b-axis thickness (Figure S26). This highlights the critical role of the shape-selective effect in TeMB formation, which is governed by the b-axis dimension. Since the kinetic diameters of TeMB molecules exceed 0.75 nm, which is significantly larger than the pore opening of ZSM-5, achieving shape-selective methylation through confinement within micropores may not be practicable. , TriMB (trimethylbenzene) usually serves as the key precursor for methylation to produce TeMB. Therefore, probing the adsorption and diffusion of TriMB isomers is essential for discovering the mechanism of the targeted durene synthesis. It is possible that smaller zeolite channels can accommodate larger molecules by inducing subcell topological flexibility in the framework, with a maximum aspect change of 15% for ZSM-5. As observed by integrated differential phase contrast scanning transmission electron microscopy (iDPC-STEM), TriMB molecules can access the pore mouths of 10MR straight channels of ZSM-5, which could be attributed to the pressure gradient across the pore mouths (Figure B). Intelligent gravimetric analysis (IGA) measurements of various TriMB isomers further confirm that 1,2,4-TriMB exhibits the highest adsorption capacity and the fastest diffusion kinetics (Figure C,D). However, due to their large size (>6.5 Å), which already approaches the topological flexibility limit of the ZSM-5 10MR straight channels (∼6.4 Å), it seems that most TriMB molecules cannot diffuse freely within the 10MR channels, as evidenced by molecular dynamics (MD) simulations (Figure S27). This indicates that TriMB molecules are largely generated at the pore mouths of the ZSM-5 zeolite via the methylation of light aromatics, predominantly xylene. Among the xylene isomers generated within the ZSM-5 micropores, para-xylene exhibits the highest diffusibility toward the pore mouths, with 1,2,4-TriMB as its primary monomethylation product. This is further corroborated by the reaction results over the InZr-4/ZSM-5­(b) catalysts, where the proportion of 1,2,4-TriMB in the total TriMB reaches 73–88% (Figure S26), surpassing the equilibrium composition of TriMB isomers. , Regarding the methylation reaction of 1,2,4-TriMB, the electron-donating and steric effects of the methyl groups on the benzene ring promote the selective positioning of the fourth methyl group at the 5-position, ultimately enabling the highly selective formation of durene. DFT calculations also demonstrate that durene is the thermodynamically most favorable product among the methylation products of various TriMB isomers (Figure S28). As a result, the shape-selective formation and targeted methylation of 1,2,4-TriMB at the pore mouths and external surfaces of ZSM-5 can be characterized as a “pore mouth shape-selective” catalytic transformation of reactant molecules larger than the zeolite pore size. Nanosheet ZSM-5, which is characterized by a high exposure of 10MR straight channels and pore mouths, assumes a crucial role in enhancing the pore mouth shape-selective effect during the methylation of benzene rings. Furthermore, the pore mouth shape-selective effect is enhanced with the increase in b-axis thickness of the ZSM-5 nanosheets. Consequently, it is imperative to precisely tailor the acidic properties and morphological parameters of ZSM-5 nanosheets to simultaneously control the number and spatial arrangement of methyl groups.

Through a systematic optimization of the Si/Al ratio and dimensional parameters of the ZSM-5 nanosheets, the InZr-4/ZSM-5­(150-90) coupled catalyst, featuring a Si/Al ratio of 150 and a b-axis thickness of 90 nm, attains a CO₂ conversion of 18.4% and a total aromatic selectivity of 75.8% (Table S16). The proportion of TeMB in the total aromatics amounts to 94.2%, with an 88.4% ratio of durene in TeMB, which significantly deviates from the thermodynamic equilibrium composition. In addition, InZr-4/ZSM-5­(150-90) delivers consistently higher TeMB/aromatics proportions than InZr-4/ZSM-5(180) across various reaction conditions (Table S17) with a durene/TeMB ratio higher than 80%, which represents the highest level reported in the literature to date (Table S18). This InZr-4/ZSM-5­(150-90) catalyst demonstrates outstanding long-term catalytic stability, with the CO₂ conversion and organic product distribution remaining similar after 100 h on stream (Figure S29 and Table S19). No significant changes in the elemental composition, coordination structure, crystal size, or spatial distribution of the InZrO _x and ZSM-5 components were detected after 100 h on stream (Figures S30–S32, Tables S6, S7, and S20). Despite the relatively low coke deposition, the coupled CO₂ hydrogenation–​aromatization–methylation reaction pathway can still be preserved. As a result, the aromatic selectivity is maintained at a high level of approximately 70%, and the product distribution profile remains largely unchanged.

Conclusions

In summary, a precisely coupled reaction pathway enabling highly selective CO₂-to-durene conversion was established through the rational design of a bifunctional catalyst comprising a tetragonal InZrO _x solid solution and ZSM-5 nanosheets. The spatial synergy between coordinatively unsaturated In–O and Zr–O sites, particularly within the In–O–Zr motifs of the optimal InZr-4 component, facilitates H₂ and CO₂ activation and promotes low-temperature CO₂ hydrogenation via the HCOO* pathway. Beyond conventional micropore-based shape selectivity, the pore mouth shape-selective effect imposed by the shallow channels or pore mouths of ZSM-5 nanosheets governs the oriented methylation of aromatic intermediates. As a result, at a relatively low temperature of 280 °C and a high aromatic selectivity of 75.8%, TeMB constituted 94.2% of the total aromatics, with durene accounting for 88.4% of the TeMB isomers. This exceptionally high durene selectivity, which markedly deviates from the thermodynamic equilibrium composition of TeMB isomers, ranks among the highest level reported so far and highlights the potential of engineering a coupling reaction pathway involving complex reaction networks toward a single target product.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.6c02583.

• Detailed experimental procedures; catalytic performances and comparison with relevant literature data; characterizations for InZr-n samples using DRIFTS, ICP-AES, XRD, Raman, HRTEM, EDS mapping, XPS, and EXAFS; characterizations for ZSM-5 samples using SEM, XRD, N₂ adsorption–desorption, NH₃-TPD, and Py-IR; calculations of the “substitution ratio of aromatic ring”; bulk and surface structures of InZr-4, InZr-2, and InZr-10 proposed from DFT calculations; energies of H₂ dissociation and CO₂ adsorption; detailed reaction pathways via HCOO* or COOH* intermediate for CO₂ hydrogenation; transition state structures of CO₂ hydrogenation; Gibbs free energy profiles of CO₂ hydrogenation and methylation of TriMB isomers; and MD simulations of the diffusion of para-xylene and TriMB isomers in MFI-structured zeolite (PDF)

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C.L., Q.L., and W.H. contributed equally.

This work was supported by the National Natural Science Foundation of China (Grant Nos. 22293025, 22293023, U25B6005, U22B6011, and 22402236).

The authors declare no competing financial interest.