Site selectivity of argon–methane co-adsorbed in mordenite as studied by NMR

Abstract

1 H spin–lattice relaxation times, T1s, for argon–methane mixtures adsorbed in Na-mordenite have been measured at 40.4 MHz from 4.2 K to room temperature. When argon is co-adsorbed with methane to ca. < 25% of the micropore capacity, the proportion of methane to argon in the main channels is almost the same as that in the side pockets, and the equilibrium population for both sites is determined by the Boltzmann distribution law. When argon is co-adsorbed approximately at the saturation limit, a sort of phase separation takes place over a wide range of temperature (4.2–100 K). In this case, ca. 90% of adsorbed methane is preferentially trapped in the side pockets, and the remaining methane, together with argon, is in the main channels. This indicates that the reorientational motion of the guest molecules can probe the very small energy difference between different adsorption sites in the mordenite pore at low temperatures.