Selective Adsorption of Polychlorinated Dibenzo-p-dioxins and Dibenzofurans by the Zeosils UTD-1, SSZ-24, and ITQ-4

Zeosils are microporous solids with a pure silica framework. Due to their hydrophobic properties, zeosils are ideal host materials for the adsorption of hydrophobic guest molecules. We tested zeosils with different pore diameters (UTD-1, SSZ-24 and ITQ-4 as well as CIT-5) for the selective adsorption of the polychlorinated dibenzo-p-dioxins and dibenzofurans. This group of highly toxic substances contains 210 congeners that possess similar chemical properties, but differ in their size and shape. In the experiment, polychlorinated dibenzo-p-dioxins and dibenzofurans were extracted from fly ash of a waste incinerator, adsorbed on amorphous silica, then thermally desorbed and flushed over a sequential arrangement of the zeosils at elevated temperature by a stream of nitrogen. ITQ-4 with the smallest pore diameter was placed first, followed by SSZ-24 and, finally, by UTD-1 with the largest pore diameter. After the experiment, the zeosils were analysed for their contents of the different congeners. The results show that the sorption of the congeners occurs selectively and that it is governed by the size and the shape of the dioxin molecules, which in turn depend on the number of chlorine atoms and the pattern of chlorine substitution (regioisomers). Geometrical reasoning as well as molecular dynamics calculations on the zeosil structures and on the dioxin molecules were helpful in rationalising the results. This work represents an especially complex case of the molecular sieving effect and may lead to a selective on-line monitoring of the concentrations of dioxin molecules in waste gases of industrial combustion processes. The size- and shape-selective sorption of dioxin molecules may also bear some resemblance to the molecular recognition process that occurs in nature at the aryl hydrocarbon receptor.