Replacing one of the protons with a Li + ion in a neighboring 6MR (denoted as 2Si-1H-8MR-1Li-6MR) allows C2H4 to diffuse without significant distortion of the 8MR (Fig. and). This substitution reduces the maximum free energy barrier for C2H4 diffusion to 70 kJ/mol. For C2H6, however, the activation free energy remains relatively high at 133 kJ/mol (Table and Fig. ).