Optimal Pore Chemistry in an Ultramicroporous Metal–Organic Framework for Benchmark Inverse CO2/C2H2 Separation

Abstract

Isolation of CO2 from acetylene (C2H2) via CO2-selective sorbents is an energy-efficient and environmentally friendly technology for C2H2 purification, but a strategic and daunting challenge due to the similarities in shape, size, and physiochemical properties of these two gases. There is still no specific methodology for the sorbent construction with CO2-preferential adsorption capability over C2H2. Herein, we demonstrate an effective strategy to construct a Ce(IV)-based ultramicroporous metal-organic framework, Ce(IV)-MIL-140-4F, for the efficient inverse CO2/C2H2 separation. The ligand-to-metal cluster charge transfer in this MOF is facilitated by the Ce(IV) ion with low-lying unoccupied 4f orbitals and tetrafluoroterephthalate linker with four electron-withdrawing F atoms, affording a perfect pore environment to match CO2 with remarkable inverse CO2/C2H2 selectivity. On the other hand, the Zr(IV)-MIL-140-4F congener exhibits conventional C2H2-selective sorption behavior. The exceptional CO2 uptake (151.7 cm3 cm-3) of Ce(IV)-MIL-140-4F along with high separation selectivities (above 40) for CO2/C2H2 mixtures set a new benchmark for inverse CO2/C2H2 separation. The unique CO2 recognition mechanism is unveiled by in situ powder X-ray diffraction experiments, Fourier-transform infrared spectroscopy measurements, and molecular calculations. Simulated and experimental breakthrough experiments comprehensively demonstrate the great potential of Ce(IV)-MIL-140-4F for efficient inverse CO2/C2H2 separation.