Effect of Fe open metal site in metal-organic frameworks on post-combustion CO2 capture performance

Iron open metal site in metal-organic frameworks, MIL-88A and MIL-127(Fe), was tested to investigate its performance on CO2 separation from a binary mixture CO2/N2 under simulated post-combustion system. The sorption experiment tests were conducted via a fixed-bed reactor with different temperatures ranging between 30°C and 60°C at atmospheric pressure. Maximum CO2 sorption capacity was observed at low temperature of 30°C, where 5.24 mmol could be adsorbed by one gram of MIL-127(Fe) and 4.95 mmol CO2 could be adsorbed by one gram of MIL-88A. The presence of moisture in the gas mixture affected CO2 sorption capacity by reducing CO2 capacity as high as 0.5 mmol CO2 per one gram sorbent. In this work, a dynamic breakthrough model was applied to investigate adsorption behavior of the metal-organic frameworks (MOFs) and of the system. The predicted model showed good agreement with experimental results; the Langmuir isotherm model fitted well with our sorption system, suggesting one molecule of CO2 is held at localized sites of the MOFs, which plausibly be at unsaturated Fe2+ metal ion site. Rate of sorption was found to increase with increasing sorption temperature for either intra-particle diffusivity or external mass transfer. Our study suggests that the MOFs containing Fe open metal sites offer an outstanding prospective for CO2 sorption and both MIL-88A and MIL-127(Fe) are good representatives applied for post-combustion CO2 capture.