Dynamics of propene and propane in ZIF-8 probed by solid-state 2H NMR.

We present a detailed 2H NMR characterization of molecular mobility of propene and propane propagating though the microporous ZIF-8, a zeolitic imidazolate framework renowned for its outstandingly high separation selectivity for industrially relevant propene/propane mixtures. Experimental characterization of both propene and propane diffusivity in ZIF-8 has been provided. Using 2H NMR spin relaxation analysis, the motional mechanisms for propene and propane guests trapped within the ZIF-8 framework have been elucidated. Kinetic parameters for each type of motion were derived. The characteristic times for microscopic translational diffusion and activation barriers (EC3H8 = 38 kJ mol-1, EC3H6 = 13.5 kJ mol-1) for propane and propene diffusivities have been estimated. A notable difference in the observed activation barriers emphasizes that the ZIF-8 window crossing is associated with the "gate-opening" and represents an extremely shape selective process. Finally, we show that the 2H NMR technique is capable of providing reliable information on microscopic diffusivity in the ZIF-8 MOF even for molecules with slow diffusivity (<10-14 m2 s-1).