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 concentration, by non-isobaric concentration pulse chromatography. At these low concentrations the heats of adsorption and Henry constants for the aromatic and olefinic hydrocarbons are substantially greater than previously reported values, which were derived from measurements at higher loading levels, while the intracrystalline diffusivities of these species are substantially smaller with higher activation energies. In contrast, the results for saturated hydrocarbons are similar to the values obtained at higher loadings. This behaviour parallels the previously reported results of detailed calorimetric measurements which, for olefins and aromatics, show a rapid increase in heat of adsorption at very low loadings, suggesting the presence of a small fraction of highly favourable adsorption sites. Such sites are probably associated with defects in the crystal structure and, although the precise nature of these sites is not established, it seems plausible to suggest that they may be associated with the presence of silanol groups.