Hierarchical ZSM‑5 Zeolites with Exceptional Mesoporosity and Catalytic Performance: A Novel Soft‑Template Approach

Abstract

We report a simple, scalable soft‑template (ST) that covalently anchors to MFI frameworks, yielding hierarchical ZSM‑5 zeolites with an external surface area up to 300 m² g⁻¹ and a narrow mesopore window of 4–8 nm. The ST, comprising a central tertiary amine linked to three short alkyl chains and three silicon atoms bearing methoxy groups, survives high‑temperature hydrothermal synthesis and prevents amorphous phase formation. Resulting particles (~1 µm) consist of 60–150 nm nanocrystals, providing short intracrystalline diffusion paths. Detailed XRD, FTIR, N₂ sorption, SEM, TEM, NH₃‑TPD, and TG/DSC analyses confirm the crystalline MFI structure, preserved acidity, and robust hydrothermal stability. The mesoporous catalysts exhibit superior performance in alkylation of benzene with benzyl alcohol, cracking of 1,3,5‑tri‑isopropylbenzene, and thermal cracking of low‑density polyethylene, achieving >90 % conversion of benzyl alcohol within 1 h and 100 % LDPE conversion at 375 °C. These results demonstrate that the ST‑induced mesoporosity unlocks the full catalytic potential of ZSM‑5 for bulky‑reactant reactions.

Background

Zeolites combine high surface acidity, large surface area, hydrothermal stability, and molecular sieving. However, their intrinsic microporosity limits diffusion of bulky molecules, impairing catalytic efficiency. While mesoporous analogues address diffusion, their amorphous frameworks compromise acidity and stability. Introducing a secondary, well‑controlled mesopore system into a crystalline zeolite—the hierarchical zeolite—offers the best of both worlds: high surface area, abundant acid sites, short diffusion paths, and robust structure.

Traditional etching approaches (dealumination, desilication) create intercrystalline pores but often degrade crystallinity and acidity. Templating strategies, especially with soft templates that can form covalent bonds with the zeolite framework, provide a safer route to mesoporosity. Yet many soft templates are complex, unstable, or prone to detachment during synthesis, limiting industrial scalability.

Our work addresses these gaps by designing a chemically stable, covalently attachable soft template that generates intracrystalline mesopores without compromising crystallinity or acidity. The resulting hierarchical ZSM‑5 catalysts outperform conventional counterparts in key bulk‑reactant processes.

Methods

Fabrication and Verification of the Soft‑Template (ST)

The ST is synthesized by reacting 3‑aminopropyltrimethoxysilane with (3‑glycidoxypropyl) trimethoxysilane in a microwave reactor under N₂ at 85 °C for 10 h, yielding C₂₄H₅₇O₁₃NSi₃ (MW = 651). FTIR confirms the expected functional groups.

Synthesis of Hierarchical ZSM‑5 Zeolites

Typical procedure: mix ST (48 wt % methanol solution) with 8.7 g silica sol and 20–60 mL TPAOH, stir to form emulsion A; prepare NaAlO₂ solution B; combine A and B, stir 3 h; hydrothermally crystallize at 80 °C (24 h) then 170 °C (3 days). Calcine at 550 °C (10 h). Optimal composition: 170 °C, 3 days, TPAOH/ST = 8. ST amounts ranged 1.3–3.9 g (samples MZ‑1 to MZ‑4). Conventional ZSM‑5 (TZ) was synthesized identically but without ST. Post‑synthesis Na⁺–exchanged samples were converted to H⁺ form by NH₄NO₃ exchange and calcination.

Characterization

Key techniques: XRD (Shimadzu XRD‑6000), FTIR (Nicolet iS50), N₂ sorption (Quantachrome Nova 2000e), SEM (Hitachi S4800), TEM (Philips FEI Tecnai G2 F20), NH₃‑TPD (Finetec Finesorb 3010), TG/DSC (Netzsch Sta 449 F3), and ICP (Varian 720) for SiO₂/Al₂O₃ ratios. Samples were degassed at 300 °C for 10 h before N₂ sorption.

Catalytic Reactions

• Alkylation: 0.30 g catalyst, 68 mL benzene, 1.0 mL benzyl alcohol, 80 °C, monitored by GC‑FID.

• Cracking of 1,3,5‑tri‑isopropylbenzene: 120 mg catalyst, 0.8 µL substrate, 300 °C, GC‑FID.

• LDPE thermal cracking: 0.0023 g H‑form catalyst, 0.023 g LDPE (≤ 500 µm, ρ = 0.925 g cm⁻³), 30–600 °C (10 °C min⁻¹), TG analysis.

Results and Discussion

Structural Integrity

All MZ samples retain the MFI crystal structure (XRD) and exhibit similar FTIR fingerprints to TZ, indicating no framework disruption by ST. N₂ sorption reveals type IV isotherms with pronounced hysteresis for MZ, confirming mesoporosity, whereas TZ shows type I.

Mesoporous Characteristics

Table 1 (summarized) shows MZ‑3 (optimal ST = 3.1 g) with S_ext = 300 m² g⁻¹, S_mic = 316 m² g⁻¹, V_mic = 0.23 cm³ g⁻¹, and a hierarchy factor (HF) of 0.18. Mesopore size distribution centers at 4–8 nm, confirming intracrystalline origin. SEM/TEM images display ~1 µm particles composed of 60–150 nm nanocrystals; no intercrystalline pores are detected.

Acidity and Thermal Stability

NH₃‑TPD shows identical weak (≈150 °C) and strong (≈375 °C) acid peaks for MZ‑3 and TZ, confirming preserved acidity. TG/DSC indicates ST combustion between 255–405 °C; after 150 °C/10 days hydrothermal treatment, microporosity slightly declines (S_mic = 218 m² g⁻¹) but the MFI framework remains intact.

Catalytic Performance

• Alkylation: MZ‑3 achieves 30 % conversion after 1 h and >90 % after 10 h, while TZ remains below 8 % and deactivates by 7 h.

• 1,3,5‑Tri‑isopropylbenzene cracking: MZ‑3 reaches 97.5 % conversion with 30.8 % benzene selectivity, far exceeding TZ (13.7 % conversion, 23.5 % benzene). The high mesoporosity facilitates rapid diffusion of bulky intermediates.

• LDPE cracking: T₅₀ for MZ‑3 is 350 °C versus 390 °C for TZ; 100 % conversion at 375 °C for MZ‑3, compared to 500 °C for TZ, demonstrating superior diffusion and coke resistance.

Conclusions

We have introduced a chemically robust, covalently attachable soft‑template that generates hierarchical ZSM‑5 zeolites with exceptional mesoporosity (S_ext = 300 m² g⁻¹) and high acidity. The intracrystalline 4–8 nm pores and nanocrystalline morphology dramatically enhance catalytic activity for bulky‑reactant processes, while preserving hydrothermal stability and recyclability. This strategy is versatile, scalable, and opens avenues for industrially relevant hierarchical zeolite catalysts.

Abbreviations

DFT:

Density functional theory

DSC:

Differential scanning calorimetry

DTG:

Derivative thermogravimetric

FTIR:

Fourier transform infrared

HF:

Hierarchical factor

HT-MZ:

Hydrothermal treated mesoporous zeolite

ICP:

Inductively coupled plasma

MZ:

Mesoporous zeolite

MZ-3-used:

The MZ‑3 catalyst used after 20 sets of cracking reaction of 1,3,5‑tri‑isopropylbenzene continuously

SEM:

Scanning electron microscopy

ST:

Soft‑template

TEM:

Transmission electron microscopy

TG:

Thermogravimetric

TPD:

Temperature‑programmed desorption

TZ:

Traditional zeolite

XRD:

X‑ray diffraction