Scalable and mesoporogen-free synthesis of a highly oriented nanorod-assembled ZSM-5 zeolite for efficient benzene alkylation with ethanol

Abstract

Hierarchical ZSM-5 zeolites with intercrystalline mesopores are crucial for improving the mass transport and accessibility of active sites. Traditional synthesis methods often require organic structure-directing agents with complicated design and/or environmentally harmful additives while involving tedious synthetic procedures. To streamline the method for constructing hierarchical ZSM-5 zeolites, we herein propose a seed-induced approach free of any additives to efficiently synthesize oriented nanorod-assembled hierarchical ZSM-5 zeolites (NA-ZSM-5) with abundant intercrystalline mesopores. By systematically examining the parameters of the seed on the resultant ZSM-5, we found that a seed with high crystallinity is a prerequisite for constructing the oriented aggregation of nanorods while the length of the nanorods can be finely controlled from ∼180 nm to ∼600 nm via adjusting the seed dosage. Importantly, the seed functions as a dynamic template on which aluminosilicate precursors gradually deposit and further evolve to c-axis oriented nanorods. The resulting NA-ZSM-5 zeolite demonstrates a relatively better catalytic lifetime of 200 h, with benzene conversion constantly higher than 53.3% and ethyl selectivity remaining above 92.4% in the benzene alkylation with ethanol. Additionally, this synthetic procedure can be successfully scaled up in 1 L and 3 L autoclaves with product yields of above 94%. This work demonstrates an efficient and scalable approach to producing hierarchical ZSM-5 zeolites with abundant intercrystalline mesopores for value-added conversion of the benzene series.

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1. Introduction

Ethylbenzene (EB) is an essential industrial feedstock with significant market demand, which is predominantly utilized in the production of styrene—a key precursor for polystyrene, acrylonitrile–butadiene–styrene, and styrene–butadiene rubber copolymers.^1,2 Currently, over 90% of global EB production is achieved via benzene–ethylene alkylation processes.³ However, alkyl raw materials are heavily reliant on catalytic cracking of naphtha, which not only accelerates the depletion of petroleum resources but also raises substantial environmental concerns. As a result, developing alternative synthetic routes has become increasingly urgent. Recent progress in coal-based chemical processes has led to an ethanol surplus, offering a promising opportunity to replace ethylene with ethanol in alkylation reactions.^4,5 This substitution can effectively suppress coke formation while prolonging catalyst lifespan, thereby making the benzene–ethanol alkylation route an attractive and sustainable alternative, drawing growing interest in both academia and industry.

Currently, ZSM-5 (MFI topology) is recognized as an effective catalyst for benzene alkylation with ethanol due to its unique pore architecture, tunable acidic properties, and excellent hydrothermal stability.^6,7 Its pore size (0.5–0.6 nm) aligns well with the molecular dimensions of EB, providing a significant selectivity advantage. However, this intrinsic microporosity of ZSM-5 imposes mass diffusion limitations, particularly in the transformation of the benzene series, during which carbon deposits caused by accumulated intermediates in micropores can adversely compromise catalytic efficiency and lifetime.^8–12

To overcome the diffusional constraints, hierarchical ZSM-5 zeolites with bimodal pore structures (micro- and mesopores) have been extensively developed for improved molecular transport and more accessible acid sites.^13–16 In this regard, traditional post-treatments with alkalis (desilication) or acids (dealumination) can produce intracrystalline mesopores, but irreversible etching of the framework often leads to decreased crystallinity, thus undermining the structural stability.^17–20 Therefore, a reasonable strategy to construct a mesoporous zeolite is the in situ formation of mesopores during crystallization periods. To this end, apart from conventional organic templates as structure-directing agents, a second template, often designed surfactant molecules, is simultaneously needed for regulating the kinetics of crystal growth and finally acquiring mesopores in zeolite crystals.^21–25 Nonetheless, the introduced second template often causes the crystal size to become larger than 1 μm, which may make inner mesopores inaccessible.

Intercrystalline mesopores originating from an assembly/stack of ZSM-5 nanocrystals have garnered increasing attention due to their easily accessible acid sites.^26,27 The morphology and dimensions of the constituent nanocrystalline ZSM-5 vary depending on the additives in the synthesis gels. For example, designed surfactants containing long hydrophobic chains and polar head groups have proved efficient at attaining a multilamellar aggregated structure, where crystal growth along the b-axis is deliberately inhibited.^28–30 Besides, some zeolite growth modifiers, such as amino acids and organosilane compounds, can suppress the ripening of protozeolitic units and subsequently foster the agglomeration of these protozeolitic nanoparticles into ZSM-5 aggregates.^31–33 These synthesis procedures often require complicated molecular design and time-consuming stepwise crystallization, which may restrict their potential applications.

Alternatively, seed-induced synthesis can streamline the tricky operational procedures for constructing ZSM-5 aggregates with intercrystalline mesopores.³⁴ Moreover, the nucleation and crystallization growth kinetics of zeolites can be finely regulated in the seeding synthesis, resulting in products with controllable morphology and porosity.^35–37 For instance, ZSM-5 nanosponges with ordered mesopore walls have been synthesized through bulk crystal seeding in multiammonium surfactant-assisted systems.³⁸ Seed-induced multilamellar ZSM-5 nanosheets were also prepared with the assistance of a specific organosilane surfactant.³⁹ Recently, a series of nanosized ZSM-5 aggregates with abundant mesopores was synthesized via a seeding method in the presence of a commercially available surfactant, i.e., CTAB, under conventional hydrothermal conditions.^40–43 In these cases, surfactants play a pivotal role in the formation of ZSM-5 aggregates through self-assembly, leading to abundant intercrystallite mesopores.^44,45 In addition to surfactants, some organic (polyhexamethylene biguanide hydrochloride) and inorganic (NaF) salts have also proved effective in providing seeding particles for the formation of stacked nanosheets and parallel rod-like crystals.^46,47 Nevertheless, the persistent use of ammonium surfactants, organosilanes and organic/inorganic salts in these seed-induced systems significantly limits their scalability due to high costs and environmental concerns.^48,49

In this work, we present a green and efficient synthesis strategy for the preparation of a c-axis-oriented nanorod-assembled hierarchical ZSM-5 via zeolite seeding without the use of surfactants or crystal growth modifiers. In this strategy, crystalline seeds play a crucial role in mediating the oriented aggregation of nanorods: first, as dynamic nucleation sites to regulate deposition of precursors, and second, as a crystallographic template to induce oriented, epitaxial crystal growth. Comprehensive investigations were conducted to illustrate the effect of gel compositions and crystallization conditions on the nanorod assembly formation. By optimizing these synthetic parameters, single-crystalline nanorod assemblies with tunable c-axis rod lengths were obtained. By directly observing the morphology and structural evolution via microscopy and spectroscopy, a plausible formation mechanism was proposed. Furthermore, under optimized conditions, the scaled-up synthesis of nanorod assemblies was successfully achieved in a 3 L autoclave, underscoring its practical applicability. The resulting nanorod ZSM-5 zeolites were evaluated in benzene alkylation with ethanol, and they exhibited superior catalytic performance due to the unique textural properties and more accessible active sites. This additive-free, seed-induced approach offers a novel pathway to kinetically regulate the formation of oriented, nanocrystal-assembled hierarchical zeolites for potential catalytic applications even on a large scale.

2. Results and discussion

2.1. Optimization of the synthesis parameters of nanorod-assembled ZSM-5

We utilized the properties of zeolite seeds to induce crystal growth to construct nanorod-assembled ZSM-5. Before focusing on the parameters of zeolite seeds in tailoring the morphology of nanorod-assembled ZSM-5, other operational conditions, such as gel compositions and crystallization conditions, have been systematically explored. Results show that conventional variables, such as the water content, NaOH concentration, and the crystallization temperature, have little effect on the overall morphology (Fig. S1 and Table S1). Under optimized conditions with the molar ratio of 1 SiO₂ : 0.01 Al₂O₃ : 0.25 NaOH : 25 H₂O, we further explored the influence of zeolite seeds on the anisotropic crystallization behaviour.

2.1.1. Crystallinity of zeolite seeds. Crystallinity of zeolite seeds is essential in seed-assisted zeolite synthesis, as seeds functioning as the nucleation sites directly influence subsequent crystal growth.^34,51 Therefore, we set the crystallization temperature low (100 °C) with different crystallization times (12, 24, or 48 h) to control the crystallinity of the ZSM-5 seeds. The powder X-ray diffraction (Fig. 1a) patterns show that the seed remains amorphous at 12 h, while characteristic MFI-type diffraction peaks emerge at 24 h and further sharpen at 48 h, indicating progressive crystallization. The ATR-IR spectra of seed samples with different crystallization times (Fig. 1b) reveal the gradual development of bands at 540 and 1218 cm⁻¹, attributed to double five-membered rings and asymmetric T–O–T vibrations, respectively.^52,53 TEM and particle size analysis (Fig. 1c–f) show an increase in the seed size from ∼20 nm to ∼200 nm with a quasi-spherical morphology. The high-resolution TEM images of zeolite seeds crystallized for 48 h (Fig. S2e and f) also reveal obvious lattice fringes. The above results collectively demonstrate the increasing crystallinity of zeolite seeds when prolonging the crystallization time.

Fig. 1 ZSM-5 nano-aggregates induced using seeds with different degrees of crystallinity. (a) XRD patterns and (b) ATR-IR spectra of three different seeds; (c) dependence of particle size on crystallization time of seeds crystallized at 100 °C. Insets: photographs of seed solutions at different crystallization times; TEM images of different seeds crystallized at 100 °C for (d) 12 h, (e) 24 h, and (f) 48 h; corresponding SEM images of MFI nano-aggregates synthesized using seeds crystallized at 100 °C for (g) 12 h, (h) 24 h, and (i) 48 h.

Zeolite seeds with different degrees of crystallinity were then subjected to synthesis gels to induce further crystallization. When the mass ratio of seed suspensions to SiO₂ was around 2 wt%, all zeolite seeds could successfully induce MFI-type crystallization (Fig. S3). Among them, only the seeds crystallized for 48 h produced well-aligned ZSM-5 nanorods (NA-ZSM-5) (Fig. 1i and S4), while seeds crystallized for a shorter time (12 h or 24 h) only formed disordered nanocrystal aggregates (Fig. 1g and h). N₂ physisorption analysis (Fig. S5 and Table S2) highlights the larger external surface area that resulted from more intercrystalline mesopores in NA-ZSM-5 compared with the disordered nanocrystal aggregates. These findings show that zeolite seeds can indeed facilitate epitaxial growth, and highly crystallized seeds can direct the anisotropic aggregation of ZSM-5 nanocrystals into hierarchical architectures.

Further analysis of the 48 h-crystallized ZSM-5 seeds reveals uniform surface protrusions (Fig. S2e and f), which correspond to coherently oriented nanoparticles. According to the oriented aggregation mechanism,^34,48,54 these protrusions serve as epitaxial nucleation sites, promoting directional crystal growth along a specific crystallographic axis. As a result, the 48 h-crystallized ZSM-5 seeds facilitate the formation of intergrown nanorods and mesoporous frameworks via anisotropic aggregation. In contrast, no visible surface protrusions are observed on the seeds crystallized for a short time (≤24 h, Fig. S2a–d), likely due to their small size and underdeveloped surface morphology. Moreover, these low-crystallinity seeds exhibit weak surface coherence, and the amorphous or proto-crystalline particles tend to aggregate randomly, driven by van der Waals forces and hydrogen bonding between surface silanol groups in the synthesis gel.^51,55 Such disordered aggregation leads to the formation of irregular, poorly aligned architectures lacking well-defined hierarchical porosity. These results clearly demonstrate that high-crystallinity seeds are essential for the successful assembly of oriented ZSM-5 nanorods and the development of ordered mesostructures.

2.1.2. Amount of seed suspensions. Having established that zeolite seeds with high crystallinity promote anisotropic crystallization, we next examined how varying seed dosage affects the morphology and size distribution of nanorod-assembled ZSM-5. A series of synthesis gels with the molar composition of 1 SiO₂: 0.01 Al₂O₃: 0.25 NaOH: 25 H₂O: x wt% seed suspension (relative to SiO₂) was prepared, where x ranges from 1 to 5 (1 wt%, 2 wt%, 3.5 wt%, and 5 wt%). Besides, controlled samples were also synthesized without adding seeds. All the samples without ZSM-5 seed suspensions added exhibit weak XRD reflections with amorphous, poorly defined particles even after 72 h of hydrothermal treatment (Fig. S6). This confirms the insufficient nucleation capacity of synthesis gels in the absence of seeds. In contrast, complete MFI-type crystallization can be induced by just 1 wt% seed suspension, as evidenced by sharp XRD peaks (Fig. S7), which underscores the indispensable role of seeds for rapid and directional framework formation.

The SEM images of samples with increasing seed contents (1–5 wt%) reveal marked differences in morphology (Fig. 2a–d). As confirmed by statistical size analysis, the average nanorod length was observed to progressively shorten from ∼604 nm (1 wt% seed addition) to ∼178 nm (5 wt% seed addition) (Fig. 2e–h). This phenomenon can be reasonably ascribed to there being more nucleation sites in the synthesis system with a denser seed concentration, which fosters the initial crystallization but limits the supplementation of silica nutrient for every epitaxial growth site, thereby constraining the elongation of single nanorods.^50,56 In addition, with the seed amount increasing, the surface area first increases and then slightly declines. Therefore, seed addition of 2 wt% is the optimal parameter considering the hierarchical structure (Fig. S8 and Table S3). These findings indicate that the seed dosage also acts as an important parameter to tune the particle size and degree of nanorod intergrowth: elongated nanorods can be obtained in synthesis gels with lower seed concentrations, while ZSM-5 assemblies with shorter nanorods need synthesis gels with higher concentrations.

Fig. 2 SEM images with different magnifications and corresponding nanorod length distributions of ZSM-5 nanorod assemblies synthesized by adding (a and e) 1 wt%, (b and f) 2 wt%, (c and g) 3.5 wt% and (d and h) 5 wt% of ZSM-5 seed suspension.

Fig. 3 presents the morphological and structural features of a representative NA-ZSM-5 sample (seed addition is 2 wt%) synthesized using the 48 h-aged seeds. The SEM and TEM images (Fig. 3a, b and S9) reveal microscale assemblies with radially aligned nanorods stacked in a bundle-like architecture, with abundant intercrystalline voids as demonstrated by N₂ sorption. Moreover, it can be discerned that these nanorods grow epitaxially from a single spheroidal core (Fig. 3c) with a diameter of ∼200 nm (marked by the dashed circle), which is nearly consistent with the seed size (Fig. S2e and f). The lattice-resolved TEM images (Fig. 3d and e) show that every nanorod bears parallel fringes with an interplanar spacing of 1.99 nm, corresponding to the (010) planes of MFI-type ZSM-5, confirming that the identically oriented growth originated from a single seed.²⁷ Selected area electron diffraction (inset in Fig. 3c) reveals that the longest dimension of the crystals runs along the c-axis direction. Energy-dispersive X-ray spectroscopy (Fig. 3f) demonstrates the homogeneous distribution of the elements Si, Al, and O within the sample. N₂ physisorption analysis (Fig. S10) reveals a type IV isotherm with a pronounced hysteresis loop in the relative pressure range of 0.4–1.0, indicative of intercrystalline mesopores. Besides, the sample also exhibits a broad pore size distribution (from 2 to 50 nm), a high BET surface area of 440 m² g⁻¹ and a mesopore volume of around 0.29 cm³ g⁻¹ (Table S2). Collectively, these observations affirm that the hierarchical structure of NA-ZSM-5 originates from epitaxial growth along the c-axis and simultaneous stacking of these nanorods.

Fig. 3 Microscopy characterization of ZSM-5 nanorod assemblies obtained in the 1 SiO₂:0.01 Al₂O₃:0.25 NaOH:25 H₂O:2 wt% seed suspension system. (a) SEM and (b–d) TEM images of ZSM-5 nanorod assemblies. The inset in (c) shows the selected area electron diffraction patterns of the area marked in the dashed square; (e) HR-TEM image of zeolite nanorods; (f) HAADF-STEM image of nanorod assembly particles and their corresponding elemental mapping.

In summary, both the crystallinity and concentration of the seed suspension greatly influence the anisotropic growth and hierarchical assembly of ZSM-5 nanorods. Highly crystalline, monodisperse seeds aged for 48 h as nucleation centers can initiate oriented aggregation and epitaxial crystal growth along the c-axis. Meanwhile, intercrystalline mesopores are formed in situ when the parallel nanorods stack up. Besides, adjusting the seed concentration is also an effective means to tailor the length of nanorods. The exact growth mechanism of the ZSM-5 nanorods needs to be thoroughly explored.

2.2. Seed-mediated formation pathway of nanorod-assembled ZSM-5

To elucidate the crystallization mechanism of the seed-induced nanorod-assembled ZSM-5, the hydrothermal synthesis processes at 170 °C were systematically tracked during a period of 0–24 h. The SEM images (Fig. S11) and XRD patterns (Fig. S12) reveal the morphological and structural evolution throughout the crystallization. During the initial induction stage (0–2 h), a dense layer of amorphous aluminosilicate precursors accumulates on the surface of the seeds, with broad signals overlapping the crystalline peaks of seeds in the XRD patterns. After 4 h of crystallization, short rod-like intermediates begin to appear. Meanwhile, the characteristic MFI reflections emerge, accompanied by a marked increase in crystallinity from 2% to 42%, indicating a rapid amorphous-to-crystalline transition. As the crystallization reaction proceeds, amorphous precursors are gradually consumed to construct ordered crystalline domains. At 12 h of crystallization, the morphology of the nanorod assemblies with well-defined crystallinity is nearly unchanged, suggesting the completion of the crystallization process.

ATR-IR analysis (Fig. S13) provides complementary structural information. Characteristic bands of the five-membered ring vibrations and asymmetric T–O–T stretching only appear in samples crystallized for at least 4 h, aligning with the changing crystallinity shown in the XRD patterns. Concurrently, the broad band at 1041 cm⁻¹ attributed to amorphous silica progressively narrows and gradually shifts to 1060 cm⁻¹, reflecting the formation of a more ordered silicate framework.^52,53 Altogether, the above characterization results witness a critical transformation window between 2 and 8 h, during which seeds initiate the nucleation and direct the formation of rod-like MFI domains.

To further investigate this key stage, TEM characterization was further conducted on samples crystallized between 2 and 8 h (Fig. 4a–d). At 2 h, amorphous precursor nanoparticles are observed accumulating on the seed surface and forming a compact nanoparticle layer. The high-resolution TEM image (Fig. S14) reveals lattice continuity between the seed and the attached pre-crystalline layer. This interfacial layer acts as an “induction layer”, which promotes oriented crystal growth along the protrusions of the seed surface. As crystallization continues, the attached nanoparticles evolve through surface reorganization and directional fusion, gradually forming short, rod-like zeolite domains (Fig. S15). Subsequently, the adsorption, rearrangement and incorporation of precursor particles along the c-axis of these domains promote these rod-like domains to elongate into mature nanorods. This epitaxial growth proceeds until the nutrition in the synthesis gels is fully depleted, at which point the oriented ZSM-5 nanorods eventually develop. Throughout this process, the c-axis-oriented growth of ZSM-5 nanorods can be clearly observed. The mechanism is likely driven by surface energy minimization in thermodynamics, whereby seed surfaces with a high curvature preferentially adsorb and reorganize precursor particles to support the anisotropic growth into surfaces with a lower curvature. This behavior aligns with prior reports on the synthesis of oriented zeolite assemblies.^46,56

Fig. 4 Low magnification (upper) and the corresponding high magnification (bottom) TEM images of intermediates at (a) 0 h, (b) 2 h, (c) 4 h and (d) 8 h of crystallization periods; (e) the proposed synthetic mechanism of hierarchical ZSM-5 nanorod assemblies.

As delineated in Fig. 4e, this process comprises three distinct stages: (1) ordered deposition of precursor nanoparticles on the seed surface, (2) fusion and reorganization of precursor nanoparticles into short rod-like domains, and (3) directional elongation into fully crystalized nanorods. It is worthwhile mentioning that this mechanism differs significantly from our previously reported CTAB-assisted system,⁵⁶ which needs to rely on the crystal growth modifier. Herein, the zeolite seed plays a dual role: as a capturing site for precursor deposition and as a crystallographic template to induce oriented, epitaxial crystal growth. Given this seed-induced strategy for constructing hierarchical ZSM-5 with intercrystalline mesopores without other environmentally polluting additives in conventional hydrothermal conditions, it is probably promising that this procedure can be scaled up for even hundred-gram level production.

2.3. Upscaling synthesis of ZSM-5 nanorod assemblies

As for the potential scaling up of hierarchical zeolites, apart from challenges such as changes in heat and mass transfer, concentration gradients, and mixing efficiency,⁵⁷ the use of expensive raw materials also increases the cost and exacerbates the environmental burden. In this context, we employed the abovementioned seed-induced route for obtaining NA-ZSM-5 and meanwhile used the relatively cheap and industrial silica source of silica gels instead of expensive tetraethyl orthosilicate, which can greatly decrease the cost of raw materials.

To evaluate the scalability of this strategy, we performed 10-fold and 30-fold upscaling synthesis in 1 L and 3 L stainless steel autoclaves (Fig. 5a), respectively, using an identical gel molar composition to that in the 100 mL laboratory batch. After hydrothermal crystallization, 71.6 g and 225.1 g of zeolite powders were obtained from the 1 L and 3 L batches, corresponding to solid yields of above 94% (Fig. 5b). The SEM images (Fig. 5c) and XRD patterns (Fig. 5d) confirm that the scaled-up products retain the characteristic nanorod morphology and the high crystallinity is almost the same as for the lab-scale sample. Based on X-ray fluorescence spectroscopy (XRF), the SiO₂/Al₂O₃ molar ratios of the 1 L and 3 L products are 47.3 and 51.3, respectively, indicating a consistent framework composition. Besides, the physicochemical properties of the scaled-up samples were also examined. The N₂ adsorption–desorption isotherms (Fig. 5e) show comparable BET surface areas, mesopore volumes, and hysteresis loop behavior to those of the laboratory-synthesized NA-ZSM-5. The NH₃-TPD profiles (Fig. 5f and Table S4) also reveal similar acid site strength and concentration. Collectively, these results confirm the reproducibility and scalability of this seed-induced synthesis strategy without any additives.

Fig. 5 The lab-scale synthesis of ZSM-5 nanorod assemblies. (a) Pictures of 100 mL, 1 L and 3 L autoclaves, (b) ZSM-5 products after calcination, (c) SEM images, (d) XRD patterns, (e) N₂ adsorption–desorption isotherms, and (f) NH₃-TPD profiles of the samples synthesized under varied scales.

2.4. Physicochemical properties of typical samples

Before evaluating catalytic performance, the physicochemical properties of NA-ZSM-5 and its counterparts (conventional coffin-like ZSM-5 and commercial ZSM-5, Fig. S16 and S17) were thoroughly examined. The N₂ adsorption–desorption isotherms (Fig. S18a) reveal that conventional coffin-like ZSM-5 and commercial ZSM-5 exhibit typical type I profiles within the relative pressure range of 0.01–0.90, indicative of well-developed microporosity. In contrast, NA-ZSM-5 displays a type IV isotherm with a pronounced hysteresis loop in the range of P/P₀ = 0.4–1.0, evidencing the presence of mesopores. Further analysis (Table S5) reveals that NA-ZSM-5 has a higher BET surface area (440 m² g⁻¹) compared to both conventional coffin-like ZSM-5 (351 m² g⁻¹) and commercial ZSM-5 (325 m² g⁻¹). Additionally, the mesopore volume of NA-ZSM-5 is significantly larger (0.29 cm³ g⁻¹) than that of conventional coffin-like ZSM-5 (0.10 cm³ g⁻¹) and commercial ZSM-5 (0.07 cm³ g⁻¹), consistent with the presence of intercrystalline mesopores formed by oriented stacking of nanorods. This increased mesoporosity in NA-ZSM-5 may be beneficial for enhancing the diffusion of reactants and products during catalytic processes. The NH₃-TPD analysis (Fig. S18b) further supports the differences in strength and concentration of the three samples. All three ZSM-5 samples show two distinct desorption peaks: one at low temperatures (200–300 °C) corresponding to weak acid sites, and another at higher temperatures (350–500 °C) corresponding to strong acid sites. Interestingly, the acid strength and concentration in NA-ZSM-5 are lower than those in commercial ZSM-5, but comparable to those in conventional coffin-like ZSM-5 (Table S5). This trend aligns well with XRF analysis, which indicates that NA-ZSM-5 has a higher SiO₂/Al₂O₃ ratio than commercial ZSM-5 but it is similar to that of conventional coffin-like ZSM-5. Together, these results highlight the unique textural properties of NA-ZSM-5, particularly the mesoporosity and relatively balanced acidity.

2.5. Catalytic performance in benzene alkylation with ethanol

To assess the catalytic performance of NA-ZSM-5, conventional coffin-like ZSM-5 and commercial ZSM-5, we selected the benzene–ethanol alkylation reaction as a probe reaction (Fig. 6a). This reaction is commonly used to evaluate the catalytic activity of zeolites due to their pore structure and sensitivity to acidity.^51,56,58 It is important to note that the NA-ZSM-5 sample used in this evaluation was synthesized with 2 wt% seed addition, which was identified as the optimal condition for achieving well-developed mesoporosity and favorable textural properties. The catalytic activity and product selectivity as a function of reaction time over the three catalysts are presented in Fig. 6b–g and Table S6. Therein, NA-ZSM-5 demonstrates significantly higher initial benzene conversion (53.3%, the average reaction results for a TOS of 10 h) compared to conventional coffin-like ZSM-5 (41.2%), even though both materials have comparable SiO₂/Al₂O₃ ratios and acid concentrations. The superior activity of NA-ZSM-5 is likely attributable to the larger surface area, which facilitates better availability of acid sites.⁵⁹ In addition, NA-ZSM-5 also shows better catalytic stability, which remains above 70% of the initial conversion even after 200 h. However, conventional coffin-like ZSM-5 undergoes a continuous drop in benzene conversion just after 25 h and can only maintain a benzene conversion of around 20% at 100 h.

Fig. 6 Long-term reaction stability of benzene alkylation with ethanol over three samples: (a) the illustration of benzene alkylation with ethanol, (b) benzene conversion, (c) ethyl selectivity, (d) toluene selectivity, (e) xylene selectivity, (f) PB selectivity and (g) selectivity towards others during reaction processes. Reaction conditions: 0.5 g of catalyst, molar ratio of benzene to ethanol is 1 : 1, temperature = 350 °C, WHSV = 6 h⁻¹, atmospheric pressure. Note: the NA-ZSM-5 sample used in this evaluation was synthesized with 2 wt% seed addition.

In terms of product selectivity (the average reaction results for a TOS of 10 h), NA-ZSM-5 also outperforms conventional coffin-like ZSM-5, with ethyl selectivity reaching 92.4%, compared to 87.0% over conventional coffin-like ZSM-5. Additionally, the selectivities for byproducts such as toluene (1.9% vs. 5.1%), xylene (0.7% vs. 1.5%), and propylbenzene (2.4% vs. 4.0%) are also lower over NA-ZSM-5, highlighting the remarkable ability to suppress undesirable side reactions. This improved stability and selectivity are likely due to the hierarchical pore structure of NA-ZSM-5, which enhances molecular diffusion and minimizes the residence time of reactants/intermediates in acidic domains, thus reducing secondary reactions such as isomerization and cracking.⁵⁶ This interpretation is further corroborated by IGA-based diffusion analysis (Fig. S19), which shows that NA-ZSM-5 indeed exhibits a markedly higher diffusion time constant (D_eff/L²), confirming its enhanced mass transport capability. Besides, compared to commercial ZSM-5 with a comparable SiO₂/Al₂O₃ of 46, the NA-ZSM-5 catalyst also exhibits better performance in benzene conversion (53.3% vs. 49.6%), ethylbenzene selectivity (92.4% vs. 88.6%) and catalyst stability. Additionally, as shown in Fig. S20, when compared to commercial ZSM-5 with a similar SiO₂/Al₂O₃ (105), under identical catalytic test conditions, NA-ZSM-5 featuring intercrystalline mesopores still demonstrates enhanced benzene conversion (53.3% vs. 50.3%), higher ethylbenzene selectivity (92.4% vs. 90.5%), and improved stability. More importantly, as shown in Fig. S21, the scaled-up NA-ZSM-5 exhibits nearly identical conversion rates and ethylbenzene selectivity to its lab-scale counterpart, demonstrating excellent scalability of this nanorod ZSM-5.

Thermogravimetric analysis (TG) was employed to evaluate the coke deposits on the catalysts after long-term tests. As shown in Fig. S22, the weight loss below 200 °C is mainly contributed by the physically adsorbed water, ethanol, benzene and aromatic products. The weight loss above 400 °C originates from the decomposition and combustion of coke species in the spent catalysts. Although NA-ZSM-5 exhibits a higher total coke content due to the longer operation time (6.4 mg g⁻¹ for 200 h), Fig. S22a shows that compared to conventional coffin-like (5.3 mg g⁻¹ for 108 h) and commercial ZSM-5 (5.9 mg g⁻¹ for 78 h), the coke deposition rate (the ratio of the mass of coke to the reaction time) on NA-ZSM-5 is still the lowest (with the same reaction time of 78 h, 0.028 mg g cat⁻¹ h⁻¹, Fig. S22b and Table S7), suggesting a synergy of the intercrystalline structure and the well-balanced acidic properties of NA-ZSM-5 on reducing the carbon deposits during the conversion of the benzene series.⁵⁶

3. Conclusion

In summary, we have developed a scalable, mesoporogen-free seeding strategy for synthesizing hierarchical ZSM-5 zeolites featuring oriented nanorod assemblies and abundant intercrystalline mesopores. The crystallinity and dosage of zeolite seeds affect the anisotropic growth, enabling precise control of nanorod length (180–600 nm) and mesostructure formation. The resulting ZSM-5 demonstrates superior catalytic performance in benzene alkylation with ethanol, achieving >53% benzene conversion, 92.4% ethyl selectivity, and stable operation over 200 h at a WHSV of 6 h⁻¹—outperforming both conventional coffin-like and commercial counterparts. Moreover, the synthesis was successfully scaled up in 1 L and 3 L autoclaves with product yields for both exceeding 94%, underscoring its industrial applicability. This green, cost-effective approach offers a promising platform for constructing hierarchical zeolites for efficient aromatic conversions.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have influenced the work reported in this paper.

Data availability

The data that support the findings of this study are available in the supplementary information (SI) of this article. Supplementary information is available. See DOI: https://doi.org/10.1039/d5qi01828e.

Acknowledgements

The authors thank the Joint Project of Dalian University of Technology–Dalian Institute of Chemical Physics (Grant No. HX20230236), Natural Science Foundation of Liaoning Province (No. 2025-BS-0030) and the Energy Revolution S&T Program of Yulin Innovation Institute of Clean Energy (No. E301190101) for supporting this work.

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