Thermal degradation of piperazine and its structural analogs

The resistance of concentrated, aqueous piperazine (PZ) to thermal degradation is a critical characteristic that recommends PZ as a solvent for amine based absorption/stripping for CO2 capture from coal fired flue gas. Seven structural analogs of PZ were thermally degraded in an effort to discover the structural reasons for the enhanced resistance of PZ. Four comparisons are developed that look at the effect of changing heteroatoms (PZ, piperidine (PD), and morpholine (Mor)), ring size in monoamines (pyrrolidine (Pyr), PD, and hexamethyleneimine (HMI)), ring size in diamines (PZ and homopiperazine (HomoPZ)), and methyl substitution (PZ, 1methylpiperazine (1-MPZ), and 2-methylpiperazine (2-MPZ)). In 6-member heterocycles with one amino function, both Mor and PD had decreased degradation rates compared to PZ, likely because the additional amino function in PZ enhances nucleophilic attack reactions. In monoamines, the expected chemical stability predicted for 5-, 6-, and 7-membered alkane rings closely predicted the thermal degradation rates of these heterocycles. Both Pyr and HMI were degraded quickly producing polymers while the 6membered PD was stable. Increasing the size of the PZ ring by the addition of one methylene group greatly enhanced the thermal degradation rate. HomoPZ loss 99% of its initial concentration after 4 weeks at 175 °C while PZ lost only 30%. Methyl substitution increased the rate of amine degradation as both 1-MPZ and 2-MPZ degraded faster than PZ at 150 °C. Unexpectedly, a methyl on the amino group (1-MPZ) enhanced degradation more than a methyl on the carbon in the α−position to the amino group (2-MPZ). An apparent first order rate constant was calculated for the thermal degradation of these structural analogs and compared to other amines of interest in CO2 capture.