Molecular mechanical investigation of the energetics of butene sorbed in H-ferrierite

The energetics and diffusion of the four butene isomers in a model of protonated ferrierite with Si/Al ratio of 8 is investigated using molecular mechanics, molecular dynamics, and a simple activated jump diffusion model, in order to determine the influence of the diffusion of the sorbent molecules onto the selectivity of ferrierite toward isobutene. Two main classes of adsorption sites are found, in the main 10-T channels and in cavities along 8-T channels. The magnitude of the self-diffusion coefficient mainly depends on the motions of the molecules in the 10-T channels, and it is found that isobutene diffuses more slowly than the linear isomers: at 623 K, D (isobutene) <0.03 × 10 cm/s, while D (trans-2-butene) ≈ 0.42 × 10 cm/s. However, the sites in the 8-T cavities act as molecular traps for linear butenes and slow down their diffusion, while they do not influence the self-diffusion of isobutene. Therefore, the diffusion of isobutene is enhanced relative to the other isomers in ferrierite, as compared with other zeolites with only one type of channels. This might be a reason for the good selectivity of ferrierite toward isobutene.