Shape selectivity in adsorption on the all-silica DD3R

The adsorption of ethane, ethene, propane, and propene on the all-silica DD3R has been investigated using the tapered element oscillating microbalance, TEOM. Single-component adsorption isotherms are for the first time reported at temperatures in the range from 303 to 473 K and at pressures up to 500 kPa. At high temperatures, the Langmuir isotherm appropriately describes the equilibrium adsorption data for ethane, ethene, and propene on the all-silica DD3R with energetically uniform sites. For the propene data below 340 K, a dual-site Langmuir model was used. Thermodynamic properties, like the isosteric heat and entropy of adsorption, have been determined. Only minor differences exist between the adsorption of ethane and ethene. Transient adsorption experiments reveal that the eight-ring windows of the all-silica DD3R are accessible to propene molecules, while they exclude propane molecules. The high shape selectivity for propene suggests that the all-silica DD3R might be effective as an adsorbent for the separation of propene and propane mixtures.