Analysis for the speciation in CO2 loaded aqueous MEDA and MAPA solution using 13C NMR technology

The competitive and cooperative reactions between the intramolecular primary and secondary amino groups in CO2 absorption have been investigated in this work. N-Methylethylenediamine (MEDA) and N-methylpropane-1,3-diamine (MAPA) were studied with various CO2 loadings at 25 °C. The 13C NMR technology was employed to obtain accurate 13C peak areas and chemical shifts for determining the concentration of each species in both of the diamine-CO2H2O systems. The results showed that the relative amounts of the species is primary-carbamate >> secondary-carbamate >  di-carbamate in the CO2 absorption process. Moreover, the relative hydrolysis order of the carbamates is di-carbamate > secondary-carbamate > primary-carbamate. The competitive and cooperative reaction mechanism was then proposed based on these results. Finally, it was found that in both the MEDA and the MAPA ternary systems, the whole CO2 absorption process can be divided into three stages: competitive stage, buffer stage and hydrolysis stage, each of which showed a different trend in the amount of each species as well as in the pH value.