Balancing the Selectivity and Capacity of Propylene via Synergistic Binding in A Nitro-Functionalized Metal-Organic Framework.

The adsorptive separation of propyne/propylene/propane (C3H4/C3H6/C3H8) is a potential alternative to energy-intensive cryogenic distillation, but it remains a formidable challenge. Here, we report the synergistic binding of propylene in a nitro-functionalized metal-organic framework, MFM-190(NO2), which exhibits an exceptional C3H6 uptake of 9.0 mmol g-1 at 298 K and 0.5 bar (conditions relevant to practical separation) with ideal adsorbed solution theory (IAST) selectivities for C3H4/C3H6 and C3H6/C3H8 at 298 K ranging from 3.3 to 4.3 and from 4.0 to 11.5, respectively. Dynamic breakthrough experiments demonstrate efficient separation of a mixture of C3 hydrocarbons, affording polymer-grade C3H6 (purity >99.9%) with a high productivity of 42.9 mol kg-1 and excellent recyclability over four consecutive cycles. In situ neutron powder diffraction, combined with infrared spectroscopy, reveals that open Cu(II) sites and pendant nitro (-NO2) groups within the pore establish jointly a complementary electrostatic environment for preferential adsorption of C3D6 over C3D8. This study highlights the unique role of nitro groups in fine tuning the balance between the adsorption selectivity and capacity of sorbent materials to tackle challenging separations.