Nuclear magnetic resonance study of Li-exchanged zeolites. I. Cation spin dynamics: 7Li in hydrated A, X, and Y zeolites

Abstract

⁷Li spin‐lattice relaxation times (T₁) were measured at 25 MHz in hydrated Li‐exchanged A, X, and Y zeolites over the temperature range between 140 and 330 K. By comparing ⁷Li 1/T₁ in hydrated (H₂O) zeolites with those in hydrated (D₂O) analogs, the contributions from the ⁷Li quadrupolar relaxation [(1/T₁)^Q_Li] and the ⁷Li–¹H dipolar relaxation [(1/T₁)^D_Li–H] were determined separately as a function of temperature. The ⁷Li quadrupole coupling constants (e²Qq) determined from (1/T₁)^Q_Li are 61.86, 72.35, and 202.8 kHz in A, X, and Y zeolites, respectively. The temperature dependence of the correlation times for ⁷Li quadrupolar relaxation (τ_Q) reveals that: (1) the dynamic state of Li cations in hydrated zeolites resembles that of the cations present in associated liquids at high concentrations; and (2) the mobility of cations increases in the order of A, X, and Y zeolites. Variations of ⁷Li e²Qq and τ_Q were explained in terms of a composite picture of the structural state of Li⁺ ions in the hydrated forms of three zeolites.