Separation of high-purity propylene through propane-selective CAU-3 isomorphs

In petrochemical processes, the separation of propylene (C3H6) from propane (C3H8)/C3H6 mixture requires high-energy cryogenic distillation procedures, owing to the similar physicochemical properties of the two components. The development of a suitable adsorbent for application in energy-saving adsorption-based separation processes has been widely investigated, with research focusing on C3H6-selective adsorbents rather than C3H8-selective ones. C3H8-selective adsorbents will enable the direct production of high-purity C3H6. Hence, herein, we report an energy-efficient C3H8-selective CAU-3 adsorbent built from the coordination bond between the [Al12(OCH3)24]12+ metal cluster and either 1,4-benzenedicarboxylate (BDC) or 2,6-naphthalenedicarboxylate (NDC) ligand. The separation performance of the two isomorphs was evaluated using a single-component adsorption isotherm and preferential C3H8 uptake over C3H6 was observed, revealing their C3H8-selective features. The evaluation of the C3H8/C3H6 selectivity was conducted using ideal gas adsorption solution theory (IAST); CAU-3-BDC showed higher C3H8/C3H6 selectivity (1.3) than CAU-3-NDC (1.2) at 298 K and 1 bar. To demonstrate the separation ability of CAU-3-BDC, a breakthrough experiment was performed on CAU-3-BDC with a 1/15 (v/v) C3H8/C3H6 gas mixture at 298 K and 1 bar. The CAU-3-BDC adsorbent can produce high-purity C3H6 (> 99.95%) under atmospheric conditions (298 K and 1 bar) and can be regenerated with a simple purge with He, as demonstrated by multi-cyclic breakthrough experiments. The binding affinity of the C3H8 molecule was determined using grand canonical Monte Carlo (GCMC) and density functional theory (DFT) calculations.