CO2 capture and conversion using Mg-MOF-74 prepared by a sonochemical method

Abstract

Mg-MOF-74 crystals were successfully prepared in 1 h by a sonochemical method (Mg-MOF-74(S)) after triethylamine (TEA) was added as a deprotonating agent. Mg-MOF-74(S) (1640 m² g⁻¹ BET surface area) displayed similar textural properties to those of a high-quality MOF sample synthesized in 24 h by the solvothermal method (Mg-MOF-74(C), 1525 m² g⁻¹). However, mesopores were formed, probably due to the competitive binding of TEA to Mg²⁺ ions, and the average particle size of the former (ca. 0.6 μm) was significantly smaller than that of the latter (ca. 14 μm). The H₂O adsorption capacity was 593 mL g⁻¹ at 298 K for Mg-MOF-74(S), displaying higher hydrophilicity than Zeolite 13X. The adsorption isotherms of Mg-MOF-74(S) for CO₂ showed high adsorption capacity (350 mg g⁻¹ at 298 K) and high isosteric heats of adsorption for CO₂ (42 to 22 kJ mol⁻¹). The breakthrough experiment confirmed excellent selectivity to CO₂ over N₂ at ambient conditions (saturation capacity of ca. 179 mg g⁻¹). Ten consecutive adsorption–desorption cycles at 298 K established no deterioration of the adsorption capacity, which showed reversible adsorbent regeneration at 323 K under helium flow for a total duration of 1400 min. Mg-MOF-74(S) also demonstrated excellent catalytic performance in cycloaddition of CO₂ to styrene oxide under relatively mild reaction conditions (2.0 MPa, 373 K) with close to 100% selectivity to carbonate, which was confirmed by GC-MS, ¹H-NMR, and FT-IR. Mg-MOF-74(S) could be reused 3 times without losing catalytic activity and with no structural deterioration.