Thermal Degradation of Piperazine/4-Hydroxy-1-methylpiperidine for CO₂ Capture

The thermal degradation of aqueous piperazine (PZ)/4-hydroxy-1-methylpiperidine (HMPD) for CO₂ capture was rigorously evaluated and compared to that of PZ/n-methyldiethanolamine (MDEA). The degradation mechanism for PZ/HMPD was investigated by performing measurements under various conditions and identifying degradation products. The "arm switching" reaction between PZ and HMPD to produce methylpiperazine (MPZ) and hydroxypiperidine (HPD) is the major degradation pathway. A second-order rate model consistent with proposed degradation pathways accurately models the initial degradation rate. The significantly greater stability of PZ/HMPD compared to that of PZ/MDEA is due to the remarkable thermal stability of its immediate degradation product, the smaller initial degradation rate of PZ/HMPD, and the greater thermal stability of HMPD compared to that of MDEA. The potential environmental issues caused by the volatility of one minor degradation product, 1,4-dimethylpiperazine (1,4 DMPZ), need to be addressed for commercial application of this solvent.