NaX Zeolites Modified with Hydrophobic Methyl Groups for Faster Thermal Charging in Water-Based Energy Storage

The optimization of water adsorption/desorption cycles in hydrophilic porous solids is critical for enhancing the efficiency of thermochemical energy storage systems. While NaX zeolites (FAU topology) possess high water adsorption capacity, their strong hydrophilic nature often results in high desorption temperatures, limiting their application in solar thermal charging. The NaX zeolites modified with methyltrimethoxysilane (MTMS) via co-condensation were investigated to create a partially hydrophobic environment around the Al(IV) sites. Samples were synthesized at MTMS/Al2O3 molar ratios of 0, 0.1, 0.2, and0.3. Results confirmed that the incorporation of MTMS does not compromise the FAU crystalline structure or significantly alter the global Si/Al ratio, indicating successful substitution within the zeolitic framework. Thermogravimetric analysis revealed that the 0.2 MTMS sample exhibited superior performance, achieving a reduction in maximum desorption temperature from 146.5 °C to 142.4 °C compared to the conventional NaX zeolite without sacrificing total water adsorption capacity. Kinetic modeling showed that all silane-modified samples demonstrated desorption rates at 40 °C approximately twice as high as the unmodified zeolite. Specifically, the 0.2 MTMS sample showed the highest desorption rate, facilitating faster thermal charging. These results demonstrate that adjusting the hydrophobic–hydrophilic balance through MTMS functionalization is a viable strategy to enhance the efficiency of zeolite-based thermal energy storage by enabling easier water expulsion at lower temperatures.