Fine-Tuning the Pore Environment of the Microporous Cu-MOF for High Propylene Storage and Efficient Separation of Light Hydrocarbons

Abstract

Ethylene (C₂H₄) and propylene (C₃H₆) are important energy sources and raw materials in the chemical industry. Storage and separation of C₂H₄ and C₃H₆ are vital to their practical application. Metal-organic frameworks (MOFs) having adjustable structures and pore environments are promising candidates for C₃H₆/C₂H₄ separation. Herein, we obtained a Cu-based MOF synthesized by H₃TTCA and pyrazine ligands. By adding different functional groups on the ligands within the MOFs, their pore environments are adjusted, and thus, the C₃H₆ storage capacity and C₃H₆/C₂H₄ separation efficiency are improved. Eventually, the fluoro- and methyl-functionalized iso-MOF-4 exhibits a better gas storage and C₃H₆/C₂H₄ separation performance compared with iso-MOF-1 (nonfunctionalized), iso-MOF-2 (fluoro-functionalized), and iso-MOF-3 (methyl-functionalized). A record-high C₃H₆ uptake of 293.6 ± 2.3 cm³ g^-1 (273 K, 1 atm) is achieved using iso-MOF-4. Moreover, iso-MOF-4 shows excellent repeatability, and only 3.5% of C₃H₆ storage capacities decrease after nine cycles. Employing Grand Canonical Monte Carlo (GCMC) simulations, it is indicated that iso-MOF-4 preferentially adsorbs C₃H₆ rather than C₂H₄ at low pressure. Single-crystal X-ray diffraction on C₃H₆-adsorbed iso-MOF-4 crystals precisely demonstrates the adsorption positions and arrangement of C₃H₆ molecules in the framework, which is consistent with the theoretical simulations. Remarkably, gas sorption isotherms, molecular simulations, and breakthrough experiments comprehensively demonstrate that this unique MOF material exhibits highly efficient C₃H₆/C₂H₄ separation. Additionally, iso-MOF-4 also possesses efficient separation of C₃H₈/CH₄ and C₂H₆/CH₄, indicating its promising potential in storage/separation of light hydrocarbons in industry.

Figure 1

Fine-tuning of pore environment through multifunctionalized ligand modification in an isoreticular MOF framework, coordination state of Cu₂(COO)₄ SBU, and coordination modes of TTCA^3–-R.

Figure 2

(a) N₂ sorption isotherms and pore size distribution for iso-MOF-1, iso-MOF-2, iso-MOF-3, and iso-MOF-4 at 77 K. (b) C₃H₆ and C₂H₄ sorption isotherms at 298 K for iso-MOF-1, iso-MOF-2, iso-MOF-3, and iso-MOF-4. (c) C₃H₆/C₂H₄ selectivity at 298 K for iso-MOF-1, iso-MOF-2, iso-MOF-3, and iso-MOF-4, calculated by the IAST method (v/v: 50/50). (d) C₃H₆ and C₃H₈ adsorption isotherms of iso-MOF-4 at 273, 298, and 303 K. (e, f) Cycles of C₃H₆ and C₃H₈ adsorption for iso-MOF-4 at 298 K.

Figure 3

Density distribution of C₃H₆ and C₂H₄ molecule mass center within iso-MOF-4 at 298 K under different pressures: (a) 10 kPa, (b) 30 kPa, and (c) 60 kPa for C₂H₄; (d) 0.5 kPa, (e) 1 kPa, and (f) 1.5 kPa for C₃H₆. C₃H₆ molecules loaded structure: (g, h) preferential binding sites for C₃H₆ molecules within the iso-MOF-4 framework, highlighted C–H···C (aromatic rings) and C–H···F interactions in pink dashed bonds. (i) Precise location of C₃H₆ in the tetragon I, pentagon II, and hexagon III regions.

Figure 4

(a) Experimental column breakthrough curves for the C₃H₆/C₂H₄ (v/v, 50/50) mixture (298 K, 1 atm) under a flow of 2.67 mL min^–1 in an absorber bed packed with iso-MOF-4. (b) Recyclability of C₂H₄ capacity on iso-MOF-4 after C₃H₆/C₂H₄ (v/v, 50/50) breakthrough tests; each separation process was carried out at 298 K and 1 atm, while regeneration was performed using He flow (100 mL min^–1) at 353 K for 30 min. (c, d) Experimental column breakthrough curves for the C₃H₈/CH₄ (v/v, 15/85) and C₂H₆/CH₄ (v/v, 15/85) mixture (298 K, 1 atm) under a flow of 4.0 mL min^–1 in an absorber bed packed with iso-MOF-4.