Proton magnetic resonance of CnH2n+2(n= 1–4) adsorbed in mordenite. Dynamic behaviour and host–guest interaction

Abstract

Proton spin–lattice relaxation times, T₁, for methane, ethane, propane and butane adsorbed in mordenite have been measured in situ over wide ranges of temperature and loading. The spin–lattice relaxation was found to be governed by the reorientation of the guest molecules in the low-temperature region. No phase transition was observed between 4.2 K and room temperature. The results also revealed that methane molecules are distributed between the main channel and the side-pocket of the host mordenite, whereas ethane, propane and butane are accommodated only in the main channel. For the molecules accommodated in the main channel the activation energy for reorientation (methane: 1.0 kJ mol^–1, ethane: 1.0 kJ mol^–1, propane: 3.5 kJ mol^–1, butane: 5.0–5.5 kJ mol^–1) and the temperature at which the T₁ minimum appears increase systematically with the size of the guest molecule, suggesting that the potential barrier for reorientation originates from the steric host–guest interaction.