Study of Confinement Effect of Extraframework Cations in Micro- and Mesoporous Materials Using solid-state Nuclear Magnetic Resonance Spectroscopy

The micro and mesoporous materials, as zeolites, have great technological impact due to the performance on chemical processes such as heterogenous catalysis, separation and gas storage, ion-exchange, organic synthesis. The presence of the aluminum atoms in the zeolite framework introduces a negative charge that is balanced by extraframework cations (M = Li⁺, K⁺, Na⁺, Ca²⁺, La³⁺, Ce^3+/4+). Depending of the exchanged cation in the zeolite can modify the chemical properties and catalytic performance. In this work, a series of zeolites (FAU, MOR and ZSM-5) were exchanged with lithium and sodium cations. The analysis tools including, Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) for elemental analysis and, X-ray Diffraction (XRD) and solid-state Nuclear Magnetic Resonance (ssNMR) were used to identify the cation location. This study demonstrates that the analysis of light extraframework cations by XRD in zeolites was unfinished because only provided information of the average cation location and the knowledge of the local structure of cationic sites in zeolites was very limited. A dehydration of zeolites results in a coordination of extraframework cations to the AlO₄⁻ tetrahedra and lead to a perturbation of the symmetry of this tetrahedron and therefore significant broadening of the ²⁷Al ssNMR signal. Also, we observed that the quadrupolar interaction decreases when increase the diameter of the cation balancing the AlO₄⁻ tetrahedron leading to the deformation is less pronounced for larger cations. Finally, this work allowed the study and characterization the nature of occupied positions from extraframweork cations in exchanged zeolites. In this work, we also studied and characterized, both directly and indirectly, the positions occupied by extraframework cations incorporated via ion exchange in different types of zeolites using ssNMR.