Enhanced 133cs Triple-Quantum Excitation in Solid-State NMR of Cs-Bearing Zeolites

TL;DR

This paper introduces enhanced pulse schemes for solid-state NMR of 133Cs, significantly improving triple-quantum excitation efficiency to better analyze cesium sites in zeolites for nuclear waste immobilization.

Contribution

Development of novel pulse schemes that double the excitation efficiency of 133Cs triple-quantum coherences in solid-state NMR.

Findings

Enhanced excitation factor of ~2 for 133Cs triple-quantum coherences.

Successful experimental verification on cesium-exchanged zeolites A and X.

Applicable to materials with small quadrupolar coupling constants.

Abstract

Geopolymers are aluminosilicate materials that exhibit effective immobilization properties for low-level radioactive nuclear waste, and more specifically for the immobilization of radioactive cesium. The identification of the cesium-binding sites and their distribution between the different phases making up the geopolymeric matrix can be obtained using solid-state NMR measurements of the quadrupolar spin 133Cs, which is a surrogate for the radioactive cesium species present in nuclear waste streams. For quadrupolar nuclei, acquiring two-dimensional multiple-quantum experiments allows the acquisition of more dispersed spectra when multiple sites overlap. However, 133Cs has a spin-7/2 and one of the smallest quadrupole moments, making multiple-quantum excitation highly challenging. In this work we present pulse schemes that enhance the excitation efficiency of 133Cs triple quantum coherences by a factor of ~2 with respect to a two-pulse excitation scheme. The improved schemes were developed by using numerical simulation and verified experimentally by applying one and two-dimensional triple-quantum solid-state NMR experiments to a mixture of cesium-exchanged hydrated zeolites A and X, which possess dynamically averaged small quadrupolar coupling constants in the order of 10 kHz.