High propene/propane selectivity in isostructural metal-organic frameworks with high densities of open metal sites.

Crystalline metal–organic frameworks (MOFs) have attracted great attention in the past decade due to their modular, tailorable structures, as well as their potential in applications such as gas storage and separations. Open metal sites in some MOFs provide special adsorption sites, which are favorable for H2 storage, [4] CO2 capture, [5] and CO2 separations. Propene is an important commercial petrochemical, produced on a large scale. Its production requires separation from propane/propene mixtures, but this is intrinsically difficult due to the similar physical and chemical properties of these molecules. Energy-intensive, costly cryogenic distillation has been used for over 70 years for this separation. In general, adsorptive separation is an energyand cost-effective alternative to distillation, and a few MOFs have shown potential for propene/propane separations through either an equilibrium-based, a kinetic-based or a gate-opening mechanism. Notably, open metal sites have been shown to play important roles in the two existing reports on the equilibrium separation of propene/propane mixtures. From a combined experimental and simulation study, Lamia et al. reported that open Cu sites in HKUST-1 interact preferentially with propene, due to specific interactions between the electron-rich p-bonding orbital in propene and the vacant sorbital of the open metal sites. Yoon et al. experimentally demonstrated that the existence of unsaturated Fe and Fe sites in MIL-100(Fe) substantially increases the strength of interaction with unsaturated propene molecules (the heat of adsorption was up to 70 kJmol 1 at low coverage), which leads to high selectivity for propene over propane (ca. 29). However, this high selectivity was observed at very low pressure (0.0025 bar), and it decreased markedly with increasing pressure presumably due to satuaration of the open metal sites. Herein, we report the adsorption selectivities of propene over propane for a series of isostructural frameworks MMOF-74 (M=Co, Mn, and Mg) with high concentrations of open metal sites. Adsorption experiments, ideal adsorbed solution theory (IAST) predictions, breakthrough experiments, first-principles calculations, and grand canonical Monte Carlo (GCMC) simulations reveal that Co-MOF-74 exhibits the highest thermodynamic propene/propane selectivity (ca. 46) ever reported for MOFs, due to strong pcomplexation between the open Co sites and the propene molecules. Remarkably, these high selectivities occur at ambient pressure and temperature, conditions favorable for industrial adsorptive separations. An unusual increase of the C3H6/C3H8 selectivity with increasing pressure is observed due to emergent behavior of the system that arises from the appropriate pore size relative to the size of the propene molecules. The series of M-MOF-74 materials is also denoted CPO-27-M or M/DOBDC (DOBDC = 2,5-dioxido1,4-benzene-dicarboxylate) in the literature. These materials have 1D hexagonal channels of 11–12 diameter with high densities of open metal sites (Figure 1) that can potentially interact with propene. Here, single-component adsorption isotherms for propene and propane were measured exper-