Aqueous and nonaqueous amine solutions as efficient sorbents for direct CO2 capture from air

The Intergovernmental Panel on Climate Change (IPCC) has identified in human influence the dominant cause of the observed warming since the mid-20th century, and today there is a general consensus about the correlation between the anthropogenic greenhouse gas (GHG) emissions in the atmosphere and the rise of the Earth's temperature. The development of strategies aimed at reducing the anthropogenic CO2 emissions, the main component of GHGs, has become a worldwide priority. Amongst them, negative emission technologies (NETs) are gaining increasing interest and, in particular, the chemical capture of carbon dioxide from ambient air (Direct Air Capture - DAC) could play a decisive role in keeping the global temperature rise well below 2°C above pre-industrial levels. DAC is an emerging technology with the potential to contrast the dispersed emissions coming from transport and residential heating, that cannot be captured at their sparse sources and that represent approximately half of the annual anthropogenic CO2 emissions. Moreover, the capture of CO2 from air is suitable to supply pure CO2 wherever it is needed. Because of the extremely low concentrations of CO2 in the ambient air (approximatively 410 ppm), the most studied sorbents for DAC are based on aqueous solutions of sodium or calcium hydroxide. These sorbents show a very high CO2 capture efficiency, but unfortunately the energy required for their regeneration is very high (calcium carbonate, formed during the absorption, is calcinated at 900-1000 °C to release CO2) and despite some recent improvements, the whole process is still too expansive. In recent papers, we have reported some experimental studies on the CO2 capture from gas mixtures by different aqueous and nonaqueous amine-based sorbents [1]. Herein, we have evaluated the possibility of transferring our previous studies to DAC technology: with the objective of developing new absorbents combining an efficient capture of ultra-diluted aerial CO2 with a lower energy demand for their regeneration compared to NaOH or KOH solutions, we have decided to investigate the performance of some amine-based sorbents in DAC systems and to correlate the CO2 capture efficiency to the chemical properties of the amines themselves. In the present work we have studied 18 different amines, selected in order to cover a wide range of different chemical structures: primary, secondary and tertiary amines, alkylamines and alkanolamines, amines with steric hindrance and diamines. Their aqueous solutions (1.5 mol dm-3) have been screened as sorbents for the direct air capture in 24-hour batch absorption experiments, carried out in a thermostatted absorption column, kept at 25°C. The CO2 capture efficiencies have been measured as a function of the absorption time by analyzing the treated air with a gas chromatograph. The carbonated species formed in solution upon the CO2 uptake and their relative amounts were evaluated by 13C NMR spectroscopy, a powerful non-invasive analytical technique that can provide valuable information on the absorption mechanism. In addition, for some particular amines, we have evaluated also the performances of their solution in organic diluents, namely in diethylene glycol monoethyl ether (DEGMEE) or in a 1:1 (volume scale) mixture of ethylene glycol (EG) and 1-propanol (PrOH): as we verified in previous studies [2], the higher solubility of CO2 in organic diluents than in water, and the lower heat capacity and vapor pressure of organic diluents compared to water, could enhance the CO2 absorption and reduce the heat required for the sorbent regeneration. The correlations between aerial CO2 absorption efficiencies, chemical structures and species formed for the different amine solutions are presented and discussed. Moreover, the CO2 capture efficiencies of the different amine solutions were compared with those obtained in the same operational condition with aqueous NaOH, sodium carbonate and potassium glycinate, some of the most commonly proposed sorbents in DAC processes.