Thermal Degradation of Aqueous Piperazine for CO2 Capture: 2. Product Types and Generation Rates

The generation of degradation products at 135 to 175 degrees C was investigated for concentrated, aqueous piperazine (PZ) loaded with CO2. From 135 to 175 degrees C, N-formylpiperazine, ammonium, N-(2-aminoethyl)piperazine, and 2-imidazolidone were found to be the most abundant products. These species accounted for 63% of nitrogen and 49% of carbon lost as PZ and CO2 during degradation. Thermal degradation of PZ is believed to be initiated by the nucleophilic attack of PZ at the alpha-carbon to a protonated amino function on H(+)PZ to create a ring opened PZ structure. H(+)PZ was found to be the active and likely initiating species required for the initial reactions of thermal degradation. Further S(N)2 substitution reactions can produce a variety of products. CO2 is not required for thermal degradation to proceed but is reduced to create formate or formyl amides, reacts with amines to form stable ureas, and dictates the overall product mix. The mechanism for CO2 reduction to formate or formyl amides is not clear but indicates the severity of thermal degradation conditions.