ADSORPTION AND DIFFUSION OF HYDROCARBONS IN SILICALITE AT VERY-LOW CONCENTRATION - EFFECT OF DEFECT SITES

Adsorption and diffusion of benzene, p-xylene, ethylbenzene, toluene, isobutene, cis-but-2-ene, trans-but-2-ene, isobutane and n-butane has been studied in large silicalite crystals, at very low sorbate concent ration, by non-isobaric concentration pulse chromatography. At these l ow concentrations the heats of adsorption and Henry constants for the aromatic and olefinic hydrocarbons are substantially greater than prev iously reported values, which were derived from measurements at higher loading levels, while the intracrystalline diffusivities of these spe cies are substantially smaller with higher activation energies. In con trast, the results for saturated hydrocarbons are similar to the value s obtained at higher loadings. This behaviour parallels the previously reported results of detailed calorimetric measurements which, for ole fins and aromatics, show a rapid increase in heat of adsorption at ver y low loadings, suggesting the presence of a small fraction of highly favourable adsorption sites. Such sites are probably associated with d efects in the crystal structure and, although the precise nature of th ese sites is not established, it seems plausible to suggest that they may be associated with the presence of silanol groups.