Researchers from the Innovation Academy for Precision Measurement Science and Technology (APM) of the Chinese Academy of Sciences have found that water can activate previously "NMR-invisible" aluminum in ultra-stable Y (USY) zeolite, a critical material in catalysis.

This activation creates synergistic acid sites that markedly improve the catalyst's performance in converting diethyl ether to ethylene. The findings were published in the Journal of the American Chemical Society.

Zeolites are microporous materials with tunable acidity, widely used in petrochemical and fine chemical industries due to their shape selectivity, activity, and stability. Water, often present in catalytic processes, influences zeolite acidity and reaction dynamics by acting as solvent, reactant, product, and accelerator. Key Lewis acid sites (LAS) include tri-coordinated framework and extra-framework aluminum species, many of which remain "NMR-invisible" and poorly understood, limiting knowledge of their catalytic roles.

To address this gap, the researchers conducted a comprehensive investigation into the dynamic interactions between "NMR-invisible" Al species and water in dehydrated USY zeolites by employing advanced solid-state NMR techniques combined with theoretical calculations. 

The results demonstrate that water undergoes dissociative adsorption on "NMR-invisible" Al sites, leading to a substantial increase of over 60% in Brønsted acid sites (BAS), as well as the formation of Brønsted/Lewis synergistic acid sites. This transformation dramatically enhances the catalytic activity of USY zeolite in the conversion of diethyl ether to ethylene. 

The evolution of Al species induced by water adsorption was monitored using one-dimensional ²⁷Al MAS NMR experiments. The water dissociation adsorption transforms "NMR-invisible" Al species into detectable forms, and facilitates the generation of BAS on tetra-, penta-, and hexa-coordinated Al. Furthermore, two-dimensional ¹H–¹H DQ–SQ NMR experiments were performed to investigate the spatial proximity of hydrogen species in the USY samples upon water adsorption. 

These results confirm the formation of Brønsted/Lewis synergistic acid sites, where newly formed Brønsted acid protons are in close proximity to Al–OH groups. Based on these findings, the researchers elucidated the interaction mechanism between water and "NMR-invisible" Al species. They proposed a water-driven regulatory mechanism that modifies zeolite active sites, offering a deeper understanding of how moisture influences catalytic behavior.

This study was supported by the Ministry of Science and Technology of China, the National Natural Science Foundation of China, and the Chinese Academy of Sciences.

2D ¹H−¹H DQ-SQ NMR spectra of water adsorbed on USY zeolites with various water loadings: 0 (a), 15 (b), 30 (c), and 45 water/u.c. (d), slices extracted at δ_DQ = 6.5 and 7.3 ppm from 2D plots (e) and schematic of B/L synergistic sites formation by water hydrolysis on LAS (Al-OH) (f). (Image by APM)