Monitoring the Diffusivity of Light Hydrocarbons in a Mixture by Magic Angle Spinning Pulsed Field Gradient NMR: Methane/Ethane/Ethene in ZIF-8

¹H magic angle spinning pulsed field gradient (MAS PFG) NMR was applied for the measurement of the diffusivities in a three-component mixture of the light hydrocarbons methane, ethane, and ethene, which were absorbed in the micropores of ZIF-8 metal-organic framework (MOF). It has been found that diffusivity in the mixture increases in the order ethane < ethene < methane with variance of self-diffusion coefficients D from 0.22 × 10^-10 m² s^-1 for ethane to 1.96 × 10^-10 m² s^-1 for methane at 313 K. This tendency in increasing of diffusivity from ethane to methane is similar to the tendency for individual hydrocarbons. It is concluded that the interaction of a hydrocarbon with the micropore wall of the ZIF-8 framework influences the mobility rather than the intermolecular hydrocarbon interaction. The latter plays no role for the molecules, when they overcome the energy barrier of the window connecting neighbor cages. The activation energies E_a for diffusivity of the hydrocarbons in mixture increases in the order methane (E_a = 2.4 kJ mol^-1) < ethene (E_a = 7.2 kJ mol^-1) < ethane (E_a = 11.5 kJ mol^-1). At the lowest studied temperature (273 K) D of methane is larger than D of ethane and ethene by 13.3 and 2.2 times, correspondingly, while the diffusivities of ethene and ethane differ by 5.8 times. This finding offers the possibility for separation of the three studied hydrocarbons on ZIF-8.