Quantum chemical studies on solvents for post-combustion carbon dioxide capture: Calculation of pKa and carbamate stability of disubstituted piperazines

Abstract

Piperazine is a widely studied solvent for post-combustion carbon dioxide capture. To investigate the possibilities of further improving this process, the electronic and steric effects of -CH3, -CH2F, -CH 2OH, -CH2NH2, -COCH3, and -CN groups of 2,5-disubstituted piperazines on the pKa and carbamate stability towards hydrolysis are investigated by quantum chemical methods. For the calculations, B3LYP, M11L, and spin-component-scaled MP2 (SCS-MP2) methods are used and coupled with the SMD solvation model. The experimental pKa values of piperazine, 2-methylpiperazine, and 2,5-dimethylpiperazine agree well with the calculated values. The present study indicates that substitution of -CH3, -CH2NH2, and -CH2OH groups on the 2- and 5-positions of piperazine has a positive impact on the CO2 absorption capacity by reducing the carbamate stability towards hydrolysis. Furthermore, their higher boiling points, relative to piperazine itself, will lead to a reduction of volatility-related losses. Capture if you can: Substituted piperazines with -CH2NH2 and -CH2OH at the 2- and 5-positions are better solvents for CO2 capture than the industrial solvent monoethanolamine (see picture). © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.