An Ultramicroporous MOF with Abundant Accessible Binding Sites for Separating MTO Products

The methanol-to-olefins (MTO) process produces ethylene (C2H4) and propylene (C3H6); however, their separation remains challenging. Herein, an ultramicroporous mixed-ligand MOF, Ni-TPT-TFSA, constructed from 2,2,3,3-tetrafluorosuccinic acid (TFSA) and 2,4,6-tris(4-pyridyl)-1,3,5-triazine (TPT) ligands with Ni ions, is reported for separating MTO products. Ni-TPT-TFSA features triangular channels decorated with triazine/pyridine rings and carboxylic acids, which enhances C3H6 adsorption affinity through C-H···O/N and C-H···π interactions. Single-component gas adsorption shows C3H6 and C2H4 uptakes of 194.77 and 60.38 cm3 g-1, respectively, at 298 K and 100 kPa. Notably, at a low pressure of 10 kPa and 298 K, the C3H6/C2H4 adsorption ratio reached 7.66, with a high separation potential (Δq) of 6.25 mmol g-1 at 100 kPa and 298 K, positioning Ni-TPT-TFSA among the benchmark MOF materials reported to date. Breakthrough experiments yield high-purity C2H4 (≥99.95%) with a productivity of 7.88 mmol g-1 at 298 K and 100 kPa, along with good durability and recyclability over multiple cycles. DFT calculations reveal that C-H···O/N hydrogen bonds and C-H···π forces in the confined pores collectively afford stronger affinity for C3H6 over C2H4. This work may provide new insights for the rational design of high-performance adsorbents for C3H6/C2H4 separation.