(350f) Cation-Exchanged LTA Zeolites for Propane / Propylene Separations

The separation of propane and propylene represents one of the most energy-intensive separations in chemical industry. As an alternative to cryogenic distillation, adsorption using microporous zeolites might offer an alternative. LTA zeolites have been shown to be promising adsorbents, but a systematic study of the effect of the extra-framework cations on propylene and propane adsorption is lacking.

In this work, we synthesized LTA zeolites exchanged with different cations (Na⁺, Li⁺, Ca²⁺, Mg²⁺, Cu²⁺, Mn²⁺, Zn²⁺). All exchanged samples were synthesized with Na⁺ as counterion and then partially exchanged with the other cation (30% and 50%). A comparison was made with pure Si-LTA. Adsorption isotherms were measured using a manometric system, coupled to a Tian-Calvet calorimeter, allowing the simultaneous measurement of the adsorption pseudo-differential enthalpy. These results were compared with DFT calculations. While Si-LTA shows no preferential adsorption of propane or propylene and is essentially heterogeneous for both probe molecules, propylene shows a strong interaction with divalent cations. Such materials open perspectives for the equilibrium-based separation of propane and propylene. On the other hand, LTA zeolites with monovalent cations (Na⁺ / Li⁺) show slow diffusion of propane, opening perspectives for a kinetic separation.