Observation of Water-Induced Synergistic Acidic Site from NMR-Invisible Al in Zeolite via Solid-State NMR Spectroscopy
Xingxing Wang, Qiang Wang, Chao Wang, Yueying Chu, Min Hu, Feng Deng, Jihong Yu, Jun Xu
Abstract
Zeolites are highly sensitive to water, which significantly affects their acidity─a key factor in catalytic reactions. This study investigates the dynamic interactions between water and often overlooked active sites, specifically the "NMR-invisible" aluminum species (tricoordinated framework Al─FAL and cationic extra-framework Al─EFAL) in ultrastable Y (USY) zeolite under ambient conditions. Using solid-state NMR spectroscopy combined with theoretical calculations, we demonstrate that water readily undergoes dissociative adsorption on these "NMR-invisible" Al sites. This process transforms both FAL and EFAL into "NMR-visible" Al species. The formation of new Brønsted acid sites on tetra-, penta-, and hexa-coordinated FAL results in an increase of over 60% in the BAS concentration in the USY zeolite. The hydrolysis of EFAL cations leads to the formation of Brønsted/Lewis acid synergistic sites, significantly improving the catalytic activity of USY zeolite. This enhancement is evident in the improved conversion of diethyl ether to ethene in the presence of moisture.