Understanding and Exploiting Window Effects for Adsorption and Separations of Hydrocarbons

The suitability of zeolites for a certain application strongly depends on their structural features. Among the types of shape selectivity, there is the still quite unexplored "cage or window effect" consisting of an unusual nonmonotonic increase of the Henry coefficient with chain length in cagelike zeolites when the guest hydrocarbon becomes too long to fit comfortably inside the wider part of the cages. This phenomenon has been addressed for alkanes in various zeolites, but a study dealing with alkenes is lacking. Because of both scientific interest and the impact on the petrochemical industry, we aimed at assessing window effects for a variety of alkenes regarding the position and number of the double bond. We used advanced molecular simulation techniques and considered the rigid all-silica channel-like OFF and cagelike ERI, CHA, and ITQ-29 zeolites. Our study reveals results similar to those of alkanes when the double bond is located at the chain extremes. Conversely, less molecular flexibility induced by intermediate positions of the double bond or the presence of more than one bond lead to a weakness of the window effect, except for the ITQ-29 because of its considerably larger cage. These findings result in significant values of this type of selectivity for separations of saturated and unsaturated hydrocarbons with chain lengths commensurate with the zeolite cages.