A calorimetric study of carbamate formation

Post combustion capture of CO2 (PCC) is currently one of the leading technologies for the reduction of green house gas emissions from power plants. The most common PCC process is based on the absorption of CO2 into aqueous amine solutions. CO2 absorption involves several parallel reactions including hydration of CO2; deprotonation of carbonic acid; protonation of the amine; and formation of carbamate. The extent to which each reaction proceeds is dependent on the conditions of absorption (or desorption), as well as the associated equilibrium constants. All reactions other than carbamate formation have been extensively investigated previously. The investigation of carbamate formation is more complex, as it cannot be studied in the absence of other, simultaneous reactions. In particular, the enthalpy of carbamate formation has been determined previously only from the temperature dependence of equilibrium constants, but this methodology is not robust. In this contribution, we use calorimetry and advanced model-based data analysis methods for the unravelling of the thermo-chemistry relevant to PCC and specifically directly determine the reaction enthalpy for carbamate formation.The reaction enthalpies of carbamate formation and amine protonation were measured in dilute aqueous solution at 298 K using isothermal titration calorimetry (ITC) for monoethanolamine (MEA), diethanolamine (DEA), and ammonia (NH3). The enthalpy of protonation was also measured for carbonate and bicarbonate. The re-determined protonation constants of the carbonate species and the three amines are in excellent agreement with previously reported results. No measured enthalpies of carbamate formation have been reported previously. For the carbamate formation reaction HCO3-+R′RNH↔R′RNCO2-, the following reaction enthalpies were determined: MEA, −29.7 ± 0.1 kJ/mol; DEA, −23.7 ± 0.9 kJ/mol, and NH3, −27.6 ± 0.9 kJ/mol. The results are in good agreement with, but much more precise and robust than estimated values reported in the literature.

Research highlights
► MEA-, DEA-, and ammonia-carbamate formation enthalpies have been measured.
► This is the first direct determination of carbamate formation enthalpies.
► Carbamate formation enthalpies follow trends in amine protonation enthalpies.