Ethane is selectively adsorbed over ethylene in their mixtures on the zeolite imidazolate framework ZIF-7. In packed columns, this results in the direct production of pure ethylene. This gas-phase separation is attributed to a gate-opening effect in which specific threshold pressures control the uptake and release of individual molecules. These threshold pressures differ for the different molecules, leaving a window of selective uptake operation. This phenomenon makes ZIF-7 a perfect candidate for the separation of olefins from paraffins, since in contrast to most microporous materials, the paraffin is selectively adsorbed. Mixture adsorption, as studied by breakthrough experiments, demonstrates that gate-opening effects can be effectively used to separate molecules of very similar size.

C2 and C3 alkanes are selectively adsorbed from mixtures over the corresponding alkenes on the zeolite imidazolate framework ZIF-7 through a gate-opening mechanism. As a result, the direct production of the pure alkene upon adsorption and the pure alkane upon desorption in packed columns is possible. Herein, a detailed investigation of the step-wise adsorption and separation of alkanes and alkenes is presented, together with a rigorous performance assessment. A molecular picture of the gate-opening mechanism underlying the unprecedented selectivity towards alkane adsorption is proposed based on DFT calculations and a thermodynamic analysis of the adsorption-desorption isotherms.

No abstract

No abstract

A new model is introduced to describe the loading dependency of diffusion in zeolites. The model is formulated around the idea of segregated adsorption in cage-like zeolites, that is, that molecules are located either in the cage or its window site. Furthermore, only the molecules located at the window site are able to make a successful jump to another cage. This so-called relevant site model (RSM) is based on the Maxwell-Stefan framework for mass transport but includes one extra parameter that describes the adsorption properties of the "relevant site". The RSM describes diffusivity data of N 2 and CO 2 in DDR (eight-ring cage-like zeolite) very well up to saturation. The observed diffusivity loading dependency is explained from the relative low window site occupancy that is typically much lower than the total occupancy at lower loadings. The model is successfully extended to nonisothermal diffusivity data of CO 2 and N 2 in DDR. Relating intermolecular correlation effects to the relevant site occupancy instead of the total occupancy leads to a quantitative prediction of the observed correlation effects and, consequently, the self-diffusivity. Analysis of the N 2 data suggests positional rearrangements in the DDR cages in a certain loading range. These effects have been incorporated in the model successfully.