Crystalline products of CO 2 capture by piperazine aqueous solutions

Crystalline products of the reactions of piperazine (PZ) solutions with CO2 sourced from the air and dry ice represent three carboxamides of piperazine, piperazinium carboxamide trihydrate (PZH+CO2−)·3H2O, piperazinium piperazine-4-carboxamide (PZH+)(PZCO2−), and bis-(piperazinium) piperazine-1,4-bis(carboxamide) trihydrate (PZH+)2(PZ(CO2−)2)·3H2O, whose crystal structures were revealed by single crystal X-ray diffraction studies. The interplay of hydrogen bonding affords the crystal stability of these hydrolytically labile piperazine carbamate derivatives which have not been observed in the solid state before. The crystal structure of piperazine monohydrate PZ·H2O is reported as well.