High-pressure low-field 1H NMR relaxometry in nanoporous materials

Abstract

A low-field NMR sensor with NdFeB permanent magnets (B0=118mT) and a pressure cell made of PEEK (4cm outer diameter) were designed for 1H relaxation time studies of adsorbed molecules at pressures of up to 300bar. The system was used to investigate methane uptake of microporous metal–organic frameworks and nanoporous activated carbon. T2 relaxation time distribution of pure methane and of methane under co-adsorption of carbon dioxide show that the host–guest interaction lead to a relaxation time contrasts, which may be used to distinguish between the gas phase and the different adsorbed phases of methane. Adsorption isotherms, exchange of methane between adsorbent particles and the surrounding gas phase, successive displacement of methane from adsorption sites by co-adsorption of carbon dioxide and CO2/CH4 adsorption separation factors were determined from the observed NMR relaxation time distributions.