Quantum Chemical Studies on Solvents for Post‐Combustion Carbon Dioxide Capture: Calculation of pK_a 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 -CH(3), -CH(2)F, -CH(2)OH, -CH(2)NH(2), -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 pK(a) values of piperazine, 2-methylpiperazine, and 2,5-dimethylpiperazine agree well with the calculated values. The present study indicates that substitution of -CH(3), -CH(2) NH(2), and -CH(2) OH groups on the 2- and 5-positions of piperazine has a positive impact on the CO(2) 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.

Fundamental Understanding of CO2Capture and Regeneration in Aqueous Amines from First-Principles Studies: Recent Progress and Remaining Challenges

“…Static QM calculations generally predict that electronwithdrawing (electron-donating) groups have a tendency to decrease (increase) basicity. 27,46,66,67 This has also been observed experimentally, where alcohol groups and alkyl groups have a tendency to decrease and increase pK a values, respectively. For example, ethylamine has a higher pK a than MEA (which contains an alcohol group).…”

“…Similarly, the substitution of electron-withdrawing CH 2 F, COCH 3 , and CN groups and electron-donating CH 3 and CH 2 NH 2 groups has a tendency to decrease and increase carbamate stability, respectively, in piperazines . In addition to electron-withdrawing/donating effects, the carbamate stability was also shown to decrease because of the increase in steric hindrance induced by some substituents, as exhibited by the increasing N–C length. , …”

“…Gangarapu et al found that methyl groups on the α-carbon of MEA exhibit little influence on basicity, but the substitution of F-containing groups on the α- and β-carbons (see Figure for the α- and β-carbon positions) have a tendency to decrease basicity, which is attributed to their electron-withdrawing effects . Similarly, electron-withdrawing groups such as CH 2 F, COCH 3 , and CN were predicted to decrease p K a values of substituted piperazines, while electron-donating groups such as CH 3 , and CH 2 NH 2 , were predicted to increase p K a values, although hydrogen bonding effects could complicate these trends …”

“…Similarly, PZCOO – and PZH + may participate in the protonation/deprotonation processes. While some studies have assessed the basicity and carbamate/bicarbonate formation energies of PZ using static QM calculations with implicit solvent, an understanding of the relative roles played by PZH + and PZCOO – were lacking. ,− …”

“…The Gibbs free energy for carbamate hydrolysis (i.e., Δ G for X COO – + H 2 O → X + HCO 3 – , where X is an amine) was predicted to decrease substantially with F-containing substituents on MEA, especially when they were placed on the β-carbon instead of the α-carbon . Similarly, the substitution of electron-withdrawing CH 2 F, COCH 3 , and CN groups and electron-donating CH 3 and CH 2 NH 2 groups has a tendency to decrease and increase carbamate stability, respectively, in piperazines . In addition to electron-withdrawing/donating effects, the carbamate stability was also shown to decrease because of the increase in steric hindrance induced by some substituents, as exhibited by the increasing N–C length. , …”