(109e) Hydrocarbon Separation in Nanopores: About Host-Guest Interactions and Molecular Freedom

Many adsorptive separation processes are based on the use of nanoporous solids. Often, molecular separation is achieved as a result of differences in energetic interactions with the framework of the adsorbent. This allows separation of mixtures of polar and apolar molecules, separation of molecules with unsaturated bonds from saturated molecules or selective adsorption of molecules with acidic or basic character. For example, the interaction between double bonds in alkenes with cations in a zeolite allows the selective removal of alkanes from alkane/alkene mixtures.

The confinement of molecules inside pores of molecular dimensions also implies significant losses of freedom. Such entropic effects counterbalance the favorable energetic attractions, but more interestingly, in some cases allow to separate molecules with very similar properties. Typically, the freedom retained by the molecules strongly depends on the degree of pore filling. As a result, separation selectivity depends strongly on operating conditions. In this paper, several examples of such entropic based separation will be discussed.