Proton mobility in solids. Part 5—Further study of proton motion in decationated near-faujasite H-sieves by pulse nuclear magnetic resonance

Abstract

Proton relaxation times have been measured at 30 and 60 MHz at temperatures between 360 and –180°C for two near-faujasite H zeolites with different iron contents, 700 and 75 p.p.m. Ammonia was removed from these samples by the “thin bed” procedure. The Fe³⁺ cations, probably on the I′ exchange sites, have an electronic relaxation time T₁(Fe)≈T₂(Fe)≈ 5 × 10^–8 s; T₁(²⁷Al)≈ 10^–2 s and T₂(²⁷Al)= 10^–5 s at 20°C. The temperature range investigated may be divided into two regions. From –180° to +180°C, the proton relaxation times T_1p and T_2p remain constant. T_1p is determined by a spin energy diffusion process, the paramagnetic impurities being the sink towards which the proton spin energy moves by a flip-flop mechanism. From 180 to 360°C, the proton longitudinal relaxation time T_1p falls sharply as the temperature is increased. Protons go from one oxygen atom to another and therefore they move to oxygen atoms which are alternately inside the hexagonal prism, inside the cubo-octahedron and inside the supercage. This motion modulates the proton-paramagnetic interaction which is shown to be the most efficient mechanism for the longitudinal relaxation.

The activation energy for the proton jumps is 19 kcal mol^–1. It is lower for partially dehydroxylated “deep bed” samples.