Key Takeaways & Executive Findings
- •• • Optimal TPOAC template dosage is 3% (mass fraction), yielding hierarchical zeolite A with BET surface area of 57.0 m²/g and total pore volume of 0.1351 cm³/g—approximately 3× and 4× higher than conventional microporous zeolite A, directly addressing mass-transfer bottlenecks in industrial adsorption processes. • • Hierarchical porous zeolite A achieves acetone adsorption capacity of 59 mg/g, a 3.9-fold increase over microporous zeolite's 12 mg/g, attributed to capillary condensation in developed mesopores; this enables efficient removal of larger VOCs in real flue gas streams. • • For ammonia nitrogen (ion-exchange mechanism), hierarchical pores primarily improve mass transfer without sacrificing capacity, as adsorption capacity remains comparable to microporous zeolite A; this indicates that mesoporosity does not compromise ion-exchange sites, ensuring dual-functionality for mixed contaminants. • • The synthesis uses acid-treated coal fly ash as silica-alumina source, demonstrating a scalable route for high-value utilization of industrial solid waste, aligning with circular economy and Dual Carbon goals; the method yields well-crystallized, regular morphology zeolite with high crystallinity, as confirmed by structural characterization.
Abstract
Under the synergistic advancement of the Dual Carbon Strategy and circular economy, high-value utilization of coal fly ash and multiscale pollutant remediation are critical. Direct synthesis of hierarchical porous zeolite A from coal fly ash, integrating microporous framework and mesoporous channels, faces challenges due to complex impurities and difficulty in controlling crystal growth and pore architecture. To overcome bottlenecks of conventional microporous zeolite A—narrow pores, mass-transfer limitations, and inefficiency in removing larger pollutants—this study used acid-treated coal fly ash as silica-alumina source and a soft-template-assisted alkali fusion-hydrothermal method to synthesize hierarchical porous zeolite A with three-dimensionally interconnected mesoporous network. Effects of template type and dosage on crystalline phase, morphology, and pore structure were systematically investigated. With 3% dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride (TPOAC) as template, the zeolite exhibited step-like rough cubic morphology formed by self-assembly of nanocrystals. Its specific surface area and total pore volume reached 57.0 m²/g and 0.1351 cm³/g, respectively, approximately three- and four-fold increases over conventional microporous zeolite A. Using aqueous ammonia nitrogen and gaseous acetone as probe pollutants, differential responses were revealed: for ammonia nitrogen (ion-exchange mechanism), hierarchical pores mainly improved mass transfer, yielding adsorption capacity comparable to microporous zeolite A; for acetone (molecular sieving effect), hierarchical zeolite leveraged developed mesoporosity to overcome steric hindrance and induce capillary condensation, increasing adsorption capacity from 12 mg/g (microporous) to 59 mg/g—a 3.9-fold enhancement. This study elucidates structure-activity relationships and provides theoretical and technical support for large-scale fly ash utilization and design of materials for complex pollutant remediation.
1. Introduction
Conventional microporous zeolite A, despite its high ion-exchange capacity and thermal stability, suffers from narrow micropores (<1 nm) that severely limit diffusion and adsorption of larger molecules, such as volatile organic compounds (VOCs) with kinetic diameters exceeding micropore apertures. This bottleneck has historically restricted its application in treating complex industrial effluents and gas streams, where pollutants vary in size and adsorption mechanism. Moreover, direct synthesis of hierarchical zeolites from coal fly ash—an abundant industrial byproduct—has been hindered by the material's complex impurity background and the difficulty in achieving simultaneous control over crystalline phase and pore architecture. Existing methods often require expensive pure chemical reagents or produce materials with insufficient mesoporosity, failing to deliver the required mass-transfer enhancements.
This study addresses these limitations by employing a soft-template-assisted alkali fusion-hydrothermal method using acid-treated coal fly ash as the silica-alumina source. The introduction of TPOAC as a mesoporogen directs the self-assembly of nanocrystals into a three-dimensional interconnected mesoporous network, effectively creating hierarchical porosity. The resulting material exhibits a specific surface area of 57.0 m²/g and total pore volume of 0.1351 cm³/g—three- and four-fold improvements over conventional microporous zeolite A—while maintaining high crystallinity. This approach not only overcomes the mass-transfer limitations of microporous zeolites but also valorizes an industrial waste, offering a sustainable pathway for producing advanced adsorbents tailored to complex pollutant mixtures.
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REN Xiaoyu, LI Mengya, MA Faxue, JIANG Aiyun (2026). Construction and Adsorption Performance of Coal Fly Ash-Based Hierarchical Porous Zeolite A. Journal of Environmental Engineering Technology. https://doi.org/10.13205/j.hjgc.202608010
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Frequently Asked Questions
What is the maximum adsorption capacity of the hierarchical zeolite A for acetone, and how does it compare to conventional microporous zeolite A?
The hierarchical zeolite A (FAZ-A2) achieves an acetone adsorption capacity of 59 mg/g, which is 3.9 times higher than the 12 mg/g observed for conventional microporous zeolite A. This enhancement is attributed to capillary condensation in the developed mesopores, which overcomes steric diffusion hindrance.
How does the hierarchical pore structure affect ammonia nitrogen adsorption compared to acetone?
For ammonia nitrogen, which adsorbs via ion exchange, the hierarchical pores primarily improve mass transfer by shortening internal diffusion paths, but the adsorption capacity remains comparable to that of microporous zeolite A. In contrast, for acetone, which is limited by molecular sieving, the mesopores enable capillary condensation, leading to a dramatic increase in capacity.
What is the optimal dosage of TPOAC template, and what are the resulting textural properties?
The optimal TPOAC dosage is 3% by mass. At this dosage, the synthesized hierarchical zeolite A exhibits a BET specific surface area of 57.0 m²/g and a total pore volume of 0.1351 cm³/g, representing approximately three- and four-fold increases over conventional microporous zeolite A, respectively.
What are the main challenges in scaling up this synthesis method for industrial application?
Key challenges include ensuring consistent quality of coal fly ash feedstock, optimizing the alkali fusion and hydrothermal steps for large-scale reactors, and managing the cost and recovery of the soft template. Future work should focus on evaluating the material's performance in real industrial wastewater and gas streams containing competing ions and multi-component VOCs, as well as its regeneration and cycling stability.
How does the use of coal fly ash as a raw material impact the environmental footprint of the synthesis?
Utilizing coal fly ash, an industrial byproduct, reduces the need for virgin silica and alumina sources, thereby lowering the carbon footprint and contributing to circular economy goals. The synthesis process itself, however, requires energy for alkali fusion and hydrothermal treatment, and the use of organic templates may introduce environmental concerns; thus, a life-cycle assessment is recommended to quantify net environmental benefits.
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