Dynamics of benzene, cyclohexane and n-hexane in KL zeolite studied by 2H NMR

Abstract

Molecular motions of benzene-d₆, cyclohexane-d₁₂ and n-hexane-d₁₄ sorbed at loading levels of 1 molecule per channel lobe in KL zeolite have been studied by ²H NMR. The spectra were recorded in the temperature range from 124 to 373 K, and they were successfully simulated. At low temperatures, benzene molecules rotate fast around the C₆ axis, and cyclohexane molecules rotate fast around the C₃ axis of the chair form, where the directions of the rotation axis are fixed. With increase in temperature, benzene, cyclohexane, and n-hexane molecules start jumping among the six equivalent sites on K⁺ ions. Further increases in temperature results in the increase in the fraction of molecules located at the central space of the micropore which undergo isotropic motions and exchange with the molecules on the K⁺ ions. The mean residence time on the K⁺ ion is in the following order: benzene-d₆>cyclohexane-d₁₂>n-hexane-d₁₄. The apparent activation energies derived from the mean residence times are 28.0±1.6 kJ mol^-1 (220 K⩽T⩽373 K) for benzene-d₆, 9.6±1.2 kJ mol^-1 (160 K⩽T⩽260 K) and 44.3±3.6 kJ mol^-1 (280 K⩽T⩽373 K) for cyclohexane-d₁₂, and about 10 kJ mol^-1 for n-hexane-d₁₄. The large activation energy at the high temperatures in cyclohexane-d₁₂ might be caused by the conformation inversion of the cyclohexane ring. The ratios of the numbers of molecules in the central space to those on the K⁺ ions are in the order of benzene-d₆<cyclohexane-d₁₂<n-hexane-d₁₄. In conclusion, the interaction between the guest molecules and KL zeolite is in the following decreasing order: benzene-d₆>cyclohexane-d₁₂>n-hexane-d₁₄.