The analysis of coking amounts for NL-ZSM-5 and ZSM-5-A/B/C during the catalytic reforming of benzene is presented in Figure 9, indicating that the parent zeolite NL-ZSM-5 exhibits the highest coking amount of 11.2%. As the TEAOH concentration and mesopore content increase, the coke amounts of ZSM-5-A/B/C decrease continuously. When the pore diameter reaches around 3.85 nm, the coke amount of ZSM-5-C zeolite during the catalytic reforming of benzene is 4.0%, which is only one-third of that of the parent sample. This isdue to the fact that intermediates such as benzene and its derivatives tend to be trapped in the microporous channels, leading to coke formation^[23].

However, the formation of mesoporous channels improves the diffusion of product molecules, thereby effectively reducing the coking rate. In addition, a main peak appears at approximately 600 °C in the DTG curve, indicating that the deposited coke is dominated by hard coke, which is difficult to be oxidized^[24]. In addition, the combined BET and Py-IR results further clarify the influence of acidity on the coking behaviour. TEAOH treatment introduces intracrystalline mesopores and hollow structures by selective desilication, which decreases the micropore fraction, and exposes more framework aluminium at the same time, leading to higher amounts of both Brønsted and Lewis acid sites, as shown in Figure 8 and Table 3. The BET and NL-DFT analyses reveal an almost linear positive correlation between the micropore fraction and the coke amount (Figure 12), indicating that the coking tendency is mainly governed by the enrichment of strong acid sites^[25−27]. In the unmodified NL-ZSM-5, many acid sites are concentrated inside narrow micropores, where aromatic intermediates experience long residence times and high local concentrations and readily undergo multi step condensation to form coke that blocks the micropores. After TEAOH modification, part of the acid sites are exposed to mesoporous and external surfaces, and the newly formed mesopores provide more open diffusion pathways, allowing intermediates and products to leave the confined regions before deep condensation occurs, so that micropore blocking coke is effectively suppressed even though the total acidity is increased.

The XRD patterns of different zeolites after catalytic reaction are illustrated in Figure 10. The MFI characteristic diffraction peak at 2θ≈23° reveals notable differences among the post-reaction samples. The diffraction peak of the NL-ZSM-5 sample, which exhibits higher carbon accumulation, shifts slightly toward lower angles. In contrast, the peaks of the TEAOH-treated samples (ZSM-5-A/B/C) progressively stabilize and shift closed to the positions observed for the parent ZSM-5. This observation suggests that carbon accumulation within the pores exerts lattice stress on the ZSM-5 crystal structure, resulting in peak displacement^[28−29]. With increasing concentrations of TEAOH treatment, the introduction of a mesoporous structure effectively reduces carbon accumulation within the micropores. This reduction mitigates interference with the crystal structure and preserves higher structural integrity. These results are consistent with the coking trends observed in the TG/DTG analysis, thus providing further evidence that TEAOH modification effectively suppresses coke deposition and helps preserve the crystalline structure.

The BET curves of different ZSM-5 zeolites after catalytic reforming reaction are listed in Figure 11, indicating that all samples exhibit a common trend. The accessible pore structure undergoes attenuation while the hierarchical characteristics remain intact. As indicated in Table 4, after the reaction, the specific surface areas of ZSM-5-NL/A/B/C decreased from 433/336/326/318 to 378/298/277/223 m²/g, respectively. The total pore volumes exhibited only minor reductions (0.24→0.20, 0.32→0.25, 0.35→0.29, 0.43→0.36 cm³/g), suggesting that the accessible pore volume of the hierarchical pores treated with TEAOH remained substantial after the reaction. For instance, ZSM-5-C retained 36 cm³/g, which is significantly higher than that of ZSM-5-NL 20 cm³/g. The most notable changes were observed in micropore volume: NL/A/B/C decreased from 0.11/0.09/0.08/0.05 to 0.05/0.04/0.03/0.02 cm³/g. The reduction quantity reaches approximately 55%, 56%, 62% and 60%, respectively. As a result, the proportion of micropores in the total pore volume after the reaction was significantly reduced from 46%, 28%, 23% and 12% to 25%, 16%, 10% and 5.6%. In contrast, the external surface area exhibited only a slight decrease (NL: 41→29, A: 38→27, B: 36→26, C: 28→22 m²/g), highlighting the more substantial reduction in specific surface area. These observations suggest that micropore accessibility was the primarily and most significantly compromised during the reaction, as it was partially obscured by deposits and accompanied by localized “coarsening”. In contrast, mesoporous channels exhibited greater resilience, continuing to serve as effective pathways for molecular diffusion after the reaction.

A significant correlation between the microporous fraction and coking behavior is evident, as illustrated in Figure 12. The coke deposition amount decreases continuously with the reduction of the micropore proportion in the zeolites, showing a significant near-linear correlation. It is important to note that the kinetic diameter of benzene is approximately 0.59 nm. It matches with the 10-membered ring pores of ZSM-5 (0.53–0.56 nm), resulting in restricted diffusion and prolonged residence time of the species within the micropores. Following alkylation, the volume of benzene further increases, rendering it more susceptible to sequential condensation and retention within the micropores. Consequently, the micropore-dominated structure is more prone to the formation of coke deposits within the pores. The introduction of mesopores through TEAOH treatment creates additional diffusion pathways, thereby mitigating the coking rate associated with this critical size matching. This explains why unmodified NL-ZSM-5 exhibits the highest level of coke formation during benzene conversion.

The XRD patterns of ZSM-5 zeolites after catalytic reactions at different temperatures are showed in Figure 13, indicting that there are significant differences in the characteristic MFI diffraction peaks of the post-reaction samples at 2θ ≈ 23°.

The NL-ZSM-5 sample, which exhibits higher carbon deposition, demonstrates a slight shift of the diffraction peak toward lower angles. In contrast, as the coke content diminished in the TEAOH-treated samples (ZSM-5-A/B/C), their peak positions progressively stabilized and were restored to near those of the parent ZSM-5. This observation suggests that carbon accumulation within the pores induces lattice stress on the ZSM-5 crystal structure, resulting in peak displacement. As the concentration of TEAOH treatment increases, the introduction of a mesoporous structure effectively reduces carbon accumulation within the micropores. This reduction mitigates interference with the crystal structure, thereby preserving greater structural integrity. These findings are consistent with the trend of coke amounts observed in TG/DTG analysis, further corroborating the beneficial role of TEAOH modification in suppressing carbon buildup and safeguarding the crystal structure.

The adsorption isotherms of ZSM-5-C zeolite at various catalytic temperatures are listed in Figure 14. The N₂ adsorption capacity demonstrates notable variations with relative pressure (p/p₀), indicating the sensitivity of the zeolite pore structure to the catalytic reaction temperature. Further analysis of the pore structure parameters in Table 5 indicates that as the catalytic reaction temperature rises from 773 to 873 K, the specific surface area (S_BET) of the sample decreases from 265 to 177 m²/g, the total pore volume (v_total) declines from 0.44 to 0.25 cm³/g, and the micropore volume (v_micro) diminishes from 0.03 to 0.01 cm³/g. This trend suggests that increasing the catalytic reaction temperature significantly accelerates the kinetic rate of the reaction system, which is manifested as more vigorous molecular motion and improved compatibility of intermolecular interactions. However, the desorption and escape of reaction intermediates or product molecules become more difficult, which directly enhances the tendency of benzene molecules and their derivatives to polymerize and form carbon deposits within the zeolite channels^[30−31].

Existing studies have elucidated the relationship between elevated temperatures and coking behavior in ZSM-5 zeolites from various perspectives. On one hand, the high-temperature environment facilitates the decomposition of carbon deposits, theoretically reducing coke blockage within the channels^[32−34]. On the other hand, the kinetic rate of catalytic reactions accelerates significantly at higher temperatures. This increase intensifies molecular motion and enhances the compatibility of intermolecular interactions, thereby complicating the desorption and escape of reaction intermediates or product molecules. As a result, the rate of coking formation within the pores escalates, exceeds the rate of coke decomposition. The detrimental effects of coke covering active sites and obstructing micropores remain predominant. Furthermore, studies indicate that when the catalytic reaction temperature of ZSM-5 zeolites exceeds 873 K, it approaches the threshold for irreversible damage to the pore framework, particularly concerning the loss of micropores. The simultaneous occurrence of pore collapse and particle sintering during the reaction directly undermines the integrity of both micropores and mesopores. This structural damage, distinct from carbon deposition effects, further exacerbates the deterioration of the pore structure^[35−36].

In summary, the porosity and specific surface area of the sample reached their minimum values following the catalytic reaction at 873 K. While elevated temperatures facilitate the decomposition of carbon deposits, this beneficial effect is insufficient to counteract the dual negative consequences of rapid carbon deposit formation and damage to the pore network framework, ultimately resulting in substantial deterioration of the pore structure. Consequently, it is crucial to balance reaction temperature and catalyst stability to avert excessive temperatures that could compromise the integrity of the catalyst structure.

The TG/DTG analysis of different reaction temperatures for ZSM-5-C are listed Figure 15, indicating that the reaction temperature significantly influenced the coke accumulation amount of the catalyst. As the reaction temperature increased from 773 to 873 K, the molecular sieve coking amount showed an increasing trend. From the perspective of thermodynamics, increasing the temperature shifts the equilibrium of the system toward the dehydrogenation-aromatization direction with a lower H/C ratio.

The micropore confinement provides additional stabilization to these more conjugated species, resulting in a stronger driving force for their formation^[37]. From a kinetic perspective, relevant steps such as oligomerization, cyclization, and hydrogen transfer exhibit pronounced tempreture dependence. Elevated temperatures accelerate formation rates. However, once larger, less polar heavy aromatic intermediates form, their desorption and diffusion within ZSM-5 channels become less favorable^[38]. This leads to longer effective residence times and slower diffusion, which is manifested as enhanced coke growth under high-temperature conditions.

Crystallographic analysis of the ZSM-5 unit cell model was performed using the Reflex module within the Materials Studio 2023 software package. As illustrated in Figure 16, the simulated XRD patterns displayed characteristic diffraction peaks at 2θ = 7.9°, 8.8°, 23.1°, 23.8° and 24.4°, which are in full agreement with those of the standard MFI topology. These findings confirm that the constructed model aligns closely with the crystal structure of the experimentally synthesized sample, thereby providing preliminary validation of the structural integrity of the fundamental model.

To further validate the reliability of the hierarchical model in describing actual adsorption behavior, CO₂ was employed as a probe molecule. Its adsorption isotherm in a hierarchical ZSM-5 model with a pore size of approximately 3 nm was simulated at 273 K. The simulated results were compared with experimental data reported by Liu and their team^[39]. As illustrated in Figure 17, the simulated adsorption amounts demonstrate a high degree ofagreement with the experimental data, effectively reproducingthe adsorption trend of CO₂ on actual molecular sieve materials.

Although the simulated values are slightly higher than the experimental values, this discrepancy can be primarily attributed to the ideal crystal model, which features a perfectly defect-free pore structure and enhanced porosity, thereby circumventing issues such as grain boundaries, pore blockage, or partial collapse that are present in actual materials. This comparison affirms the reliability of the constructed multi-pore model regarding its adsorption performance.

Detailed model construction steps, force field parameters, simulation ensembles, and calculation procedures are provided in the Supporting Information. In summary, the above analyses confirms the structural accuracy and performance reliability of the simulation model employed in subsequent studies of benzene molecule adsorption and diffusion behavior, thereby establishing a foundation for the credibility of the entire simulation research.

To assess the adsorption performance of benzen in hierarchical ZSM-5 molecular sieves, the adsorption isotherms and corresponding adsorption enthalpies were calculated under varying pore sizes at temperatures of 773, 823 and 873 K. As illustrated in Figure 18, the equilibrium adsorption amount of benzene exhibits an inverse relationship with the mesopore size at a constant temperature. This trend is in full agreement with experimental BET measurements, which show a decrease in micropore volume with increasing TEAOH treatment intensity. This suggests that the micropores serve as the main sites reponsible for benzene adsorption. The partial destruction of the micropore structure during mesopore formation directly reduces the effective micropore volume available for benzene adsorption and diminishes high-energy adsorption sites, resulting in an overall decrease in adsorption capacity. At constant pressure, the equilibrium adsorption capacity of benzene significantly declines with rising temperature. This phenomenon occurs because benzene adsorption on ZSM-5 is an exothermic physical adsorption process, and higher temperatures impede the adsorption process. At elevated temperatures, benzene molecules gain sufficient kinetic energy to desorb more readily from adsorption sites, hence a decline in their equilibrium adsorption capacity in the pores. This trend is consistent with fundamental thermodynamic principles that govern adsorption processes.

The adsorption heat curves of benzene at various temperatures on ZSM-5 molecular sieves with differing pore sizes are shown in Figure 19, indicating that the adsorption heat exhibits a consistent decrease with increasing temperature in the 3.0 nm mesoporous model. This phenomenon can be attributed to the heterogeneous energy distribution of adsorption sites on the solid surface. At lower temperatures, molecules can occupy multiple sites, including a significant number of weakly bound locations, leading to an overall average measurement of adsorption heat. As the temperature increases, molecules preferentially desorb from these weakly bound sites, ultimately leaving only those bound to the highest-energy sites. Consequently, the reduced adsorption heat observed at elevated temperatures does not imply a universal weakening of all interactions, but rather indicates that the population of occupied adsorption sites becomes concentrated toward those with higher energies. Nonetheless, even at these high-energy sites, vigorous molecular thermal motion results in an apparent binding energy that is lower than its value at lower temperatures. Furthermore, at a constant temperature, the adsorption heat value exhibits a slight decrease with increasing pore size. This trend indicates that in the minimal-pore ZSM-5 model (2.0 nm), benzene molecules experience the most pronounced spatial confinement effects, thereby their van der Waals interactions with the pore walls are maxmized. The introduction of mesopores generates diffusion pathways while simultaneously diluting this highly confined environment, resulting in benzene molecules being, on average, situated within an adsorption environment characterized by relatively weaker interactions^[40].

The adsorption density distribution of benzene on ZSM-5 zeolites across various pore sizes and temperatures are listed in Figure 20. A comparison of the density distributions for the same pore size model at different temperatures (773–873 K) reveals a distinct trend. An increase in temperature leads to decrease in adsorption density distribution. At 773 K, benzene molecules form localized region of high-density within confined spaces, such as pore intersections, of the 3.0 nm mesopore model.

As the temperature rises to 873 K, enhanced molecular thermal motion increases the kinetic energy of the molecules, enabling them to readily escape from the strongest adsorption sites. This observation is consistent with the finding in Section 2.3.2 that adsorption heat diminishes with rising temperature, thereby indirectly demonstrating that elevated temperatures reduce the average interaction forces between benzene molecules and the pore channels.

As the mesopore diameter increases from 2.0 to 3.0 nm, the spatial distribution characteristics of benzene molecules within the pore channels exhibit significant alterations. In ZSM-5 with a mesopore diameter of 2.0 nm, benzene molecules display a marked spatial aggregation effect. Density contour plots indicate a high concentration of benzene at pore junction regions (~0.55 nm) and the sinuous bends of the sinusoidal channels. This nanoscale spatial confinement effect results in notably elevated local concentrations, creating adsorption “hotspot” regions. Furthermore, GCMC simulations suggest that the majority of benzene adsorption occurs in micropores, thereby demonstrating that micropores retain their primary role in adsorption.

The MSD-T curves of benzene in ZSM-5 for different pore sizes and temperatures are presented in Figure 21, indicating that the diffusion of benzene molecules is jointly influenced by temperature and pore size. When the temperature is raised from 773 to 873 K in the 3.0 nm pore model, the diffusion coefficient increases from 6.95 × 10⁻⁵ to 2.77 × 10⁻⁴ cm²/s, representing a fourfold increase. This trend is in accordance with the principle that higher temperatures provide molecules with greater kinetic energy, thereby enhancing their migratory capacity. Conversely, at a constant temperature of 823 K, a reduction in pore size from 3.0 to 2.0 nm leads to a significant decrease in the diffusion coefficient from 1.48 × 10⁻⁴ to 3.74 × 10⁻⁵ cm²/s, corresponding a 75% reduction. This phenomenon suggests that although temperatures facilitated diffusion, the influence of pore sizes reduction on hindering on mass transfer is even more pronounced. A smaller pore size not only intensifies interactions between molecules and pore wall surfaces but also increases the complexity of diffusion pathways, collectively slowing their overall transport through the channel system. The synergistic effects of temperature and pore size indicate that pore dimensions exert a more significant constraint on diffusion behavior under high-temperature reaction conditions. This finding reveals the intricate nature of mass transfer in hierarchical ZSM-5 catalysts and provides essential insights into coking mechanisms. Although high temperatures enhance molecular diffusion capacity. narrower pore structures considerably prolong the molecular residence time within the channels, thereby establishing the probability of deep reactions and coke formation. These results underscore that catalyst design must carefully balance the synergistic interplay between temperature and pore size, which requires the strategic optimization of pore architecture to achieve an optimal trade-off between reaction efficiency and coking resistance.

By combining macroscopic and microscopic analyses, this work deciphers the synergistic effects of reaction temperature and pore structure on coke formation during benzene conversion over ZSM-5 catalysts. In previous research, the reduction in carbon deposition has predominantly been attributed to enhanced diffusion resulting from the introduction of mesopores. However, simulations and BET data from this study indicate that a reduction in adsorption sites is equally critical. First, the rise in mesoporosity results in a systematic decrease in both the adsorption amount and adsorption heat of benzene. This indicates that TEAOH treatment, in creating mesopores, partially damages the micropore structure which hosts high-energy adsorption sites—an effect often considered a structural “sacrifice”. Nevertheless, this “sacrifice” yields significant kinetic advantages. the diffusion coefficient of benzene molecules in larger pores (e.g., 3.0 nm) increases nearly fourfold compared to that in confined environments (e.g., 2.0 nm) (1.48 × 10⁻⁴ vs. 3.74 × 10⁻⁵).

Therefore, this study posits that the anti-coking advantage of mesopores arises from a moderate sacrifice of strong adsorption capacity for reactant molecules, which is compensated by a substantial enhancement of their diffusivity. The anti-coking mechanism encompasses two aspects. 1—Diffusion enhancement mechanism: Mesopores serve as a “highway” for molecular transport, significantly reducing the residence time of benzene molecules and their derivatives within the pore channels, thereby allowing them to diffuse away before undergoing deeper reactions. 2—Adsorption site regulation mechanism: Although the etching of some micropores does not reduce the total number of strong acid sites—indeed, Py-IR data show the total Brønsted acidity increases from 0.28 to 0.67 mmol/g upon modification—it does reduce the proportion of those strong sites located within highly confined micropore environments that are prone to initiating deep condensation. At the same time, the L/B ratio decreases from 1.57 for NL-ZSM-5 to 0.91 for ZSM-5-C (Table 3), which has a higher fraction of Brønsted sites. This favors protonation and alkylation routes that yield readily desorbed, light alkyl-aromatics, while suppressing dehydrogenation-condensation pathways on Lewis acid sites associated with extra-framework aluminum, thereby reducing the formation of highly unsaturated polyaromatic coke precursors. At the same time, the relocation of strong acid sites cooperates with the hierarchical pore network in regulating both adsorption strength and residence time. Molecular simulations show that micropores provide the strongest adsorption environment for benzene, whereas mesopores have weaker adsorption enthalpies but much higher diffusion coefficients. When strong acid sites are concentrated in micropores, the combination of strong adsorption and severe confinement leads to long residence times and high local concentrations, which promotes deep dehydrogenation and condensation. After TEAOH treatment, a larger fraction of strong acid sites is situated on mesoporous and external surfaces. In these more open regions, the weaker confinement and faster diffusion shorten the residence time of benzene derived intermediates at strong acid sites, thereby reducing their probability of evolving into polyaromatic coke and slowing down catalyst deactivation. Additionally, the micropore volume decreased from 0.11 to 0.05 cm³/g, representing a reduction of nearly 50%, which further compresses the physical space available for “micropore strong acid sites prone to inducing condensation.” This modification fundamentally diminishes the likelihood of carbon deposit precursor formation.

Molecular simulations indicate a significant increase in the diffusion coefficient of benzene at elevated temperatures (873 K). However, thermogravimetric analysis (TGA) results demonstrate a sharp increase in carbon deposition. This seemingly contradictory phenomenon may be attributed to the competition between reaction kinetics and mass transfer dynamics under high temperatures. According to the Arrhenius law, while higher temperatures improve molecular diffusivity, they exert a far stronger accelerating effect on surface reaction rates, including protonation, dehydrogenation, and condensation. That is, at high temperatures, once a benzene molecule is adsorbed, the rate at which it undergoes a series of reactions to transform into a poorly desorbable coke precursor far exceeds its rate of diffusive escape. Consequently, coke deposition on the zeolite catalyst increases to 6.9% under these conditions. Therefore, the essence of high-temperature carbon deposition is the dominanct role of ultra-fast surface reaction kinetics, whose negative impact completely overshadow the marginal positive effects brought by enhanced diffusion.

Based on the above discussion, it is evident that pore size and temperature are not isolated factors; rather, they function as interdependent regulators that co-govern the coking behavior of the catalyst. This study demonstrates that an optimized pore structure is key to resisting the negative effects of high temperatures. In the pure micropore catalyst (NL-ZSM-5), slow diffusion combined with rapid reactions at high temperatures resulted in the most severe coking (11.9%). In contrast, in the hierarchical catalyst (ZSM-5-C), the efficient diffusion channels allow effective molecular transport even at high temperatures, thereby significantly reducing residence time and thereby keeping the carbon deposition at a low level (4.12%).

In summary, this study concludes that the anti-deposition advantage of hierarchical ZSM-5 arises not only from the space-release effect contributed by mesopores, but more importantly from the synergistic regulation of “adsorption-diffusion” initiated by the restructuring of micropores and mesopores. This finding offers substantial guidance for the design of hierarchical catalysts. A deliberate modification strategy should incorporate adequate mesoporous networks to ensure superior diffusion properties while retaining essential micropores to sustain adsorption capacity, thus achieving an optimal balance between catalytic activity and stability.