Computational investigation of the nitrosation mechanism of piperazine in CO2 capture.

Quantum chemistry calculations and kinetic modeling were performed to investigate the nitrosation mechanism and kinetics of diamine piperazine (PZ), an alternative solvent for widely used monoethanolamine in postcombustion CO2 capture (PCCC), by two typical nitrosating agents, NO2- and N2O3, in the presence of CO2. Various PZ species and nitrosating agents formed by the reactions of PZ, NO2-, and N2O3 with CO2 were considered. The results indicated that the reactions of PZ species having NH group with N2O3 contribute the most to the formation of nitrosamines in the absorber unit of PCCC and follow a novel three-step nitrosation mechanism, which is initiated by the formation of a charge-transfer complex. The reactions of all PZ species with NO2- proceed more slowly than the reactions of PZ species with ONOCO2-, formed by the reaction of NO2- with CO2. Therefore, the reactions of PZ species with ONOCO2- contribute more to the formation of nitrosamines in the desorber unit of PCCC. In view of CO2 effect on the nitrosation reaction of PZ, the effect through the reaction of PZ with CO2 shows a completely different tendency for different nitrosating agents. More importantly, CO2 can greatly accelerate the nitrosation reactions of PZ by NO2- through the formation of ONOCO2- in the reaction of CO2 with NO2-. This work can help to better understand the nitrosation mechanism of diamines and in the search for efficient methods to prevent the formation of carcinogenic nitrosamines in CO2 capture unit.