Nano-engineered zeolites represent a promising class of materials for hydrogen storage via physisorption, leveraging their well-defined microporous structures, thermal stability, and tunable surface chemistry. The effectiveness of zeolites in hydrogen storage depends on several factors, including pore-size distribution, cation exchange, and operating temperature. These materials compete with metal-organic frameworks (MOFs) and activated carbons, each offering distinct advantages and limitations in hydrogen storage applications.

Zeolites are aluminosilicate frameworks with uniform pore sizes typically ranging from 0.3 to 1.5 nanometers. Their crystalline structure allows precise control over pore dimensions, which is critical for optimizing hydrogen physisorption. The interaction between hydrogen molecules and the zeolite surface is governed by weak van der Waals forces, making pore-size tuning essential for maximizing storage capacity. Studies indicate that zeolites with pore diameters close to 0.7 nm exhibit enhanced hydrogen uptake due to overlapping potential fields from opposite pore walls, creating stronger adsorption sites. For example, zeolite A (LTA) with a pore size of 0.41 nm shows a hydrogen storage capacity of approximately 1.5 wt% at 77 K and 1 bar, while zeolite X (FAU) with larger pores (0.74 nm) achieves up to 2.0 wt% under the same conditions.

Cation exchange is another critical factor influencing hydrogen storage in zeolites. The presence of extra-framework cations such as Na+, K+, or Li+ alters the electrostatic environment within the pores, enhancing the binding energy of hydrogen molecules. For instance, replacing Na+ with Li+ in zeolite X increases the isosteric heat of adsorption from 5 kJ/mol to 7 kJ/mol, leading to improved low-pressure uptake. However, excessive cation loading can reduce pore volume and accessibility, necessitating a balance between electrostatic enhancement and porosity preservation. Experimental data show that Li-exchanged zeolite X achieves a hydrogen uptake of 2.3 wt% at 77 K and 1 bar, outperforming its Na+ counterpart.

Temperature plays a pivotal role in zeolite-based hydrogen storage. Physisorption is highly temperature-dependent, with optimal performance observed at cryogenic temperatures (77 K). At ambient temperatures, the storage capacity drops significantly due to reduced adsorption enthalpy. For example, zeolite Y (FAU) exhibits a hydrogen uptake of 1.8 wt% at 77 K but less than 0.3 wt% at 298 K under the same pressure. To mitigate this limitation, researchers are exploring hybrid systems combining zeolites with high-enthalpy adsorbents or integrating them with compression technologies for practical applications.

Comparisons with MOFs and activated carbons highlight the trade-offs between these materials. MOFs excel in surface area and pore volume, with some variants like MOF-5 achieving hydrogen uptakes exceeding 7 wt% at 77 K and 20 bar. Their modular synthesis allows precise control over pore geometry and chemical functionality, but they often suffer from lower thermal and mechanical stability compared to zeolites. Activated carbons, on the other hand, offer cost advantages and moderate surface areas (up to 3000 m²/g), with hydrogen storage capacities around 3 wt% at 77 K and 20 bar. However, their amorphous pore structures lead to less predictable performance compared to crystalline zeolites or MOFs.

The table below summarizes key performance metrics for these materials:

Material Surface Area (m²/g) Pore Size (nm) H₂ Uptake (wt%, 77 K, 1 bar)
Zeolite X (Li+) 800 0.74 2.3
MOF-5 3800 1.2 4.5
Activated Carbon 2500 0.5-2.0 2.0

Despite their lower absolute storage capacities, zeolites offer advantages in terms of scalability, cost, and robustness. Their industrial familiarity from catalysis and gas separation applications facilitates integration into existing infrastructure. Recent advances in nano-engineering, such as hierarchical pore structuring and surface functionalization, aim to bridge the performance gap with MOFs while maintaining zeolites' inherent stability.

Ongoing research focuses on optimizing zeolite compositions and morphologies for hydrogen storage. For example, introducing mesopores into microporous frameworks improves kinetics and accessibility, while doping with transition metals enhances hydrogen spillover effects. Computational modeling aids in predicting optimal zeolite configurations for targeted performance, reducing the need for empirical trial-and-error.

In summary, nano-engineered zeolites present a viable pathway for hydrogen storage, particularly in applications where stability and cost are critical. While they may not match the peak capacities of MOFs, their tunable porosity, cation-mediated binding, and industrial compatibility make them a compelling option. Future developments in material design and hybrid systems could further enhance their role in the hydrogen economy.