On the mechanisms of CO₂ absorption and desorption with phase transitional solvents

Two phase transitional aqueous solvent blends consisting of diethylenetriamine (DETA) and N,N,N′,N′,N″-pentamethyldiethylenetriamine (PMDETA) were experimentally investigated to determine their CO₂ absorption, phase transition, and desorption performance. Both solvents revealed higher CO₂ loading capacities and desorption pressures than a 5 M monoethanolamine solution under comparable conditions. The evolution of solvent species during the CO₂ absorption and desorption processes was determined quantitatively by using ¹H, ¹³C, and 2-D nuclear magnetic resonance spectroscopic techniques. Mechanisms of the CO₂ absorption and desorption reactions were proposed based on the speciation analysis. Carbon dioxide absorption occurred as the carbamates and dicarbamates of DETA were formed, followed by CO₂ hydration promoted by proton exchange with PMDETA. Desorption of CO₂ could generally be described as a reverse absorption process, but it was also coupled with a dual phase transition.