Site selectivity of methane adsorbed in mordenite as studied by NMR

Abstract

Temperature dependence of the ¹H spin–lattice relaxation time, T₁, of methane adsorbed in Na-mordenite has been measured in situ at low temperatures and at various coverages. A sample free from paramagnetic impurities has revealed that at low methane loadings methane molecules were distributed in the main channels and side-pockets approximately according to the Boltzmann distribution, and reorient with activation energies of 1.0 and 2.0 kJ mol^–1, respectively. The difference in potential energies for adsorption between the main channel and the side-pocket was very small and estimated to be 70 ± 5 J mol^–1. It was also found that the rotational tunnelling of guest methane affects the proton spin–lattice relaxation at low temperatures.