Exploring the membrane-based separation of CO2/CO mixtures for CO2 capture and utilisation processes: Challenges and opportunities

The separation and removal of CO2 from its mixtures with CO is gaining increasing interest due to the novel processes in which these two gases are mixed, such as the non-thermal plasma activated reaction of CO2 splitting, a promising CO2 utilisation route that could be performed using renewable energy. The aim of this review is to propose a novel database suitable for membrane scientists to evaluate the feasibility of membrane-based separation processes involving such gas mixture, not included in the original Robeson’s works on the upper bound, nor in later developments. For this reason, we reviewed the data on the permeation, diffusion and sorption of these two gases in different classes of polymers, from polyolefins to polyimides and green polymers, spanning over a wide range of permeability values. Furthermore, we propose an upper bound for this separation, and provide a theoretical explanation for it. The separation mechanism is solubility-driven, and all polymeric membranes inspected in the literature show a CO2-selective behaviour, despite a very limited, or unfavourable, diffusion selectivity for CO2, which is consistent with empirical correlations. Consequently, the observed selectivity values are determined by the solubility-selectivity and are comprised mainly in the range 7–20, in agreement with known empirical correlations between the solubility and the critical temperature of the penetrants. Temperature has a detrimental effect on CO2/CO selectivity, as the activation energy for permeation of CO2 is always lower than that of CO. In general, while the permeability can vary over several orders of magnitude depending on the polymer nature, selectivity mostly ranges between 7 and 20, which makes the trade-off mechanism between permeability and selectivity rather weak in the case of this mixture. Such an effect provides a wider variety of design choices, and makes this separation attractive for polymeric membranes, if carried out at low temperatures and with CO2-philic materials. A preliminary calculation of the separation obtainable with single-stage membrane unit for a binary mixture is carried out for some representative polymers.