Thermodynamics and kinetics of CH4/CO2 binary mixture separation by metal-organic frameworks from isotope exchange and adsorption break-through

Abstract

We combine the isotope exchange technique (IET) with the conventional Sieverts method, microcalorimetry, breakthrough experiments and multi-scale modelling to comprehensively and quantitatively investigate the thermodynamics and kinetics of equimolar CH4/CO2 mixture separation in metal organic frameworks. The prototypical MOF-5 is selected for this work as it allows benchmarking our binary mixture results with the pure gas data widely reported in the literature. For the first time, an experimental binary gas adsorption isotherm of CH4/CO2 on MOF-5 is reported and compared with the respective pure gas isotherms. The equilibrium thermodynamic selectivity from the IET experiments for the equimolar CH4/CO2 separation is found to be 8.3 while a much lower value of 2.83 is obtained from the ideal adsorption solution theory (IAST). The large standard deviation of the model selectivities and the significant deviation of averaged model selectivity from the experimental one clearly reinforces the necessity to determine the selectivity reliably using experiments. The kinetic selectivity for the binary mixture separation determined by combining the results of IET with the linear driving force (LDF) model is 0.73. The co-adsorption heats and excess uptake of both gases in mixtures are lower than those of pure gases; we observe that the intrinsically weaker sorption of CH4 on MOF-5 is further weakened by the presence of strongly interacting CO2. Thermodynamic and kinetic selectivities and the co-adsorption heats quantitatively suggests that CH4/CO2 separation is driven by the equilibrium thermodynamic factors with no significant contribution from kinetic factors.