Towards sustainable biogas upgrading: MIL-120(Al) as a cost-effective water stable MOF for CO2/CH4 separation

Abstract

The microporous MIL-120(Al) MOF has been tested for CO₂/CH₄ separation. The material has been synthesised in powder form at the kilogram scale and shaped into spherical beads without significant loss in adsorption capacity (on average, ca. 7% for CO₂). MIL-120(Al) is more selective towards CO₂ (IAST mean selectivity of 10.6–35 between 0.1–10 bar), showing a good adsorption capacity and a moderate enthalpy of adsorption (−36.7 to −39.1 kJ mol⁻¹ in the low loading regime). Molecular simulation studies have revealed a probable rotation of the µ₂-OH groups with increasing CO₂ pressure altering the CO₂ adsorption capacity at higher pressures. Breakthrough experiments have confirmed the CO₂/CH₄ selectivity of MIL-120(Al) and have shown that the material can retain its original adsorption separation performance after regeneration, following consecutive cycles (10 cycles) and prolonged exposure to high amounts of water vapour, demonstrating the potential of this MOF for biogas upgrading.