Jr. Hufton et al., ADSORPTION AND DIFFUSION OF HYDROCARBONS IN SILICALITE AT VERY-LOW CONCENTRATION - EFFECT OF DEFECT SITES, Microporous materials, 5(1-2), 1995, pp. 39-52
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.