Quasi-Discrete Pore Engineering via Ligand Racemization in Metal-Organic Frameworks for Thermodynamic-Kinetic Synergistic Separation of Propylene and Propane.

The adsorptive separation of propylene and propane offers an energy-efficient alternative to the conventional cryogenic distillation technology. However, developing porous adsorbents with both high equilibrium and kinetic selectivity remains extremely challenging due to the similar size and physical properties of these gases. Herein, this work reports a ligand racemization strategy to construct quasi-discrete pores in MOFs for a synergistically enhanced thermodynamic and kinetic separation performance. The use of enantiopure l-malic acid versus racemic dl-malic acid as ligands afforded isoreticular Ni-based MOFs with contrasting one-dimensional channels (l-mal-MOF) and quasi-discrete cavities connected by small windows (dl-mal-MOF). The periodic pore constrictions in dl-mal-MOF significantly increased the differentiation in diffusion rates and binding energies between propylene and propane. dl-mal-MOF exhibited an exceptional propylene uptake of 1.82 mmol/g at 0.05 bar and 298 K along with an ultrahigh equilibrium-kinetic combined selectivity of 62.6. DFT calculations and MD simulations provided insights into the synergistic mechanism of preferential propylene adsorption and diffusion. Breakthrough column experiments demonstrated the excellent separation and high-purity recovery of propylene over propane on dl-mal-MOF. The robust stability and facile regeneration highlight its potential for propylene purification applications.