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

Molecular motions of benzene-d(6), cyclohexane-d(12) and n-hexane-d(14) sorbed at loading levels of 1 molecule per channel lobe in KL zeolite have been studied by H-2 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 C-6 axis, and cyclohexane molecules rotate fast around the C-3 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-d(6)> cyclohexane-d(12)> n-hexane-d(14). The apparent activation energies derived from the mean residence times are 28.0 +/- 1.6 kJ mol(-1) (220 K less than or equal to T less than or equal to 373 K) for benzene- d(6), 9.6 +/- 1.2 kJ mol(-1) (160 K less than or equal to T less than or equal to 260 K) and 44.3 +/- 3.6 kJ mol(-1) (280 K less than or equal to T less than or equal to 373 K) for cyclohexane-d(12), and about 10 kJ mol(-1) for n- hexane-d(14). The large activation energy at the high temperatures in cyclohexane-d(12) 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-d(6)< cyclohexane-d(12)< n-hexane-d(14). In conclusion, the interaction between the guest molecules and KL zeolite is in the following decreasing order: benzene-d(6)> cyclohexane-d(12)> n-hexane-d(14).