Direct CO2 capture from air with aqueous and nonaqueous diamine solutions: a comparative investigation based on 13C NMR analysis

Five different diamines, namely 1,2-diaminopropane, N,N-diethylethylenediamine, piperazine, 1-methylpiperazine and N,N,N',N'-tetramethylethylenediamine, have been investigated as liquid sorbents for the direct capture of CO2 from air (DAC – Direct Air Capture) in both aqueous and non-aqueous solutions, using a 1:1 (volume scale) mixture of ethylene glycol and 1-propanol as an organic diluent. The capture performance of the 1.5 mol dm−3 diamine solutions was evaluated on the basis of the CO2 absorption efficiency from an atmospheric air stream, continuously measured by means of a gas chromatograph during 24-hour bench-scale absorption experiments, carried out in an absorption column maintained at 25°C. 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 reaction mechanisms. The correlations between the aerial CO2 absorption efficiency, the chemical structure of diamines and the carbonated species formed in their different solutions during capture were presented and discussed. Furthermore, the CO2 capture performance in DAC processes of the diamine solutions was compared to that of some of the more conventional alkanolamine solutions under the same operating conditions.