MOLECULAR MECHANICAL INVESTIGATION OF THE ENERGETICS OF BUTENE SORBEDIN 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 mole cular mechanics, molecular dynamics, and a simple activated jump diffu sion model, in order to determine the influence of the diffusion of th e sorbent molecules onto the selectivity of ferrierite toward isobuten e. Two main classes of adsorption sites are found, in the main 10-T ch annels and in cavities along 8-T channels. The magnitude of the self-d iffusion coefficient mainly depends on the motions of the molecules in the 10-T channels, and it is found that isobutene diffuses more slowl y than the linear isomers: at 623 K, D (isobutene) < 0.03 x 10(-4) cm( 2)/s, while D (trans-2-butene) approximate to 0.42 x 10(-4) cm(2)/s. H owever, the sites in the 8-T cavities act as molecular traps for linea r butenes and slow down their diffusion, while they do not influence t he self-diffusion of isobutene. Therefore, the diffusion of isobutene is enhanced relative to the other isomers in ferrierite, as compared w ith other zeolites with only one type of channels. This might be a rea son for the good selectivity of ferrierite toward isobutene.