Structure and Framework Association of Lewis Acid Sites in MOR Zeolite

Zeolites, although key materials used in industrial processes, remain poorly understood on a molecular level despite their well-defined crystal lattices. In fact, obtaining a direct spectroscopic signatures and resolving the structure of Lewis acid sites (LAS) has remained a challenge. In this work, thanks to 1D and 2D ¹H, ¹⁵N, and ²⁷Al MAS NMR spectroscopy, carried out at different temperatures (from 298 down to 107 K), we were able to obtain the NMR spectroscopic signatures of LAS and Brønsted acid sites (BAS) in mordenite zeolite in the presence and the absence of adsorbed pyridine (Py). Combined with DFT modeling, this information enabled the structure of LAS to be revealed, namely (≡SiO)₃Al sites interacting with pyridine, thus indicating that the corresponding base-free framework-associated sites are pseudo tricoordinated Al sites, namely tricoordinated Al sites interacting with an additional coordinated adjacent siloxane bridge. With this information in hand, we propose a molecular-level understanding on how the Al^IV and Al^VI framework and framework-associated sites evolve upon dehydration and exposure to Py into BAS and LAS, and their associated Py adducts. By measuring and analyzing the changes in quadrupolar coupling constants (C_Q) that reflect electrical charge distribution around the nuclei, we further show that the lower C_Q values observed at 298 K are due to residual dynamics that makes the electric field around aluminum nuclei more symmetric. Thus, NMR spectroscopic signatures of ²⁷Al greatly vary with temperature; this information illustrates the importance of accounting for the temperature effect when confronting experimental and calculated C_Q values of the corresponding aluminum sites in zeolites in order to obtain accurate structural assessment.