An Ultramicroporous Metal-Organic Framework for High Sieving Separation of Propylene from Propane

Abstract

Highly selective adsorptive separation of olefin/paraffin through porous materials can produce high purity olefins in a much more energy-efficient way than the traditional cryogenic distillation. Here we report an ultramicroporous cobalt gallate metal-organic framework (Co-gallate) for the highly selective sieving separation of propylene/propane at ambient conditions. This material possesses optimal pore structure for the exact confinement of propylene molecules while excluding the slightly large propane molecules, as clearly demonstrated in the neutron diffraction crystal structure of Co-gallate⊃0.38C₃D₆. Its high separation performance has been confirmed by the gas sorption isotherms and column breakthrough experiments to produce the high purity of propylene (97.7%) with a high dynamic separation productivity of 36.4 cm³ cm^-3 under ambient conditions. The gas adsorption measurement, pore size distribution, and crystallographic and modeling studies comprehensively support the high sieving C₃H₆/C₃H₈ separation in this MOF material. It is stable under different environments, providing its potential for the industrial propylene purification.

Figure 1.

Structure of Co-gallate MOF and rationale for C₃H₆/C₃H₈ separation. (a) The complexity of different hydrocarbon separation systems classified by the molecular size difference. (b) The crystal structure of Co-gallate MOF without guest; purple, red, gray, and white nodes represent Co, O, C, and H atoms, respectively. (c) The coordination mode of the gallate linker. (d) Schematic diagram of the size sieving separation for C₃H₆ and C₃H₈ molecules. Co-gallate MOF shows elliptical pore window with dimensions of 5.1 and 4.2 Å, respectively, which are slightly larger than those of C₃H₆ molecule (5.1 and 4.1 Å, respectively) and smaller than those of C₃H₈ molecule (5.3 and 5.1 Å, respectively).

Figure 2.

Gas sorption properties of Co-gallate. (a) Single-component sorption isotherms of carbon dioxide at 195 K, nitrogen at 77 K. The inset shows the pore size distribution of Co-gallate MOF (about 5.2 Å) calculated from 77 K N₂ adsorption isotherm based on the Horvath–Kawazoe model. (b) Gas sorption isotherms of propylene and propane at 298 K for Co-gallate. (c) Comparison of propane uptake with reported porous materials.^,,,, (d) Qualitative comparison of adsorption IAST selectivity with uptake of different porous materials for an equimolar propylene/propane mixture.^,,,, IAST selectivity of the MOF sieves is largely uncertain associated with the low uptake of propane. Ideal molecular sieves here are defined as those which can completely block C₃H₈ molecules and take up large amount of C₃H₆ molecules from gas mixtures.

Figure 3.

Neutron diffraction crystal structure of Co-gallate·0.38C₃D₆ and preferential binding sites for C₃D₆. (a) The packing diagram of the C₃D₆ absorbed structure. The preferential binding sites for C₃D₆ (site I, site II, and site III) are represented in blue, green, and pink, respectively. The framework and the pore surface are shown in pale gold and yellow. (b) View of the optimal aperture of Co-gallate MOF for exact confinement of propylene molecules. The light blue and gray spheres represent H and C atoms of propylene molecules. (c–e) Three different preferential adsorption sites, site I (c), site II (d), and site III (e), and their close contacts with the framework, with C─D⋯O, C─D⋯π, and O─H⋯π interactions highlighted as red, black, and orange dashed lines, respectively.

Figure 4.

Column breakthrough results of Co-gallate MOF. (a) Breakthrough curves for Co-gallate MOF for an equimolar binary mixture of C₃H₆/C₃H₈ at 298 K and 1 bar. The breakthrough experiments were carried out in a packed column with 1.26 g sample at a flow rate of 1.7 mL min⁻¹. The points are experimental data, and the lines are drawn to guide the eye. (b) Concentration curve of the desorbed C₃H₆ from Co-gallate during the regeneration process. (c) Dynamic adsorption capacity of Co-gallate for C₃H₆ with five breakthrough experimental cycles. (d) Multicomponent breakthrough curves for a quaternary mixture of CH₄/C₂H₆/C₃H₆/C₃H₈ (5/5/45/45) at 298 K and 1 bar. The points are experimental data, and the lines are drawn to guide the eye.