Carbamate Stabilities of Sterically Hindered Amines from Quantum Chemical Methods: Relevance for CO₂ Capture

Abstract

The influence of electronic and steric effects on the stabilities of carbamates formed from the reaction of CO2 with a wide range of alkanolamines was investigated by quantum chemical methods. For the calculations, B3LYP, M11-L, MP2, and spin-component-scaled MP2 (SCS-MP2) methods were used, coupled with SMD and SM8 solvation models. A reduction in carbamate stability leads to an increased CO2 absorption capacity of the amine and a reduction of the energy required for solvent regeneration. Important factors for the reduction of the carbamate stability were an increase in steric hindrance around the nitrogen atom, charge on the N atom and intramolecular hydrogen bond strength. The present study indicates that secondary ethanolamines with sterically hindering groups near the N atom show significant potential as candidates for industrial CO2-capture solvents.

Quantum Chemical Studies on Solvents for Post‐Combustion Carbon Dioxide Capture: Calculation of pKa and Carbamate Stability of Disubstituted Piperazines

“…A relatively poor correlation ( R 2 =0.89) was observed between the experimental p K a values and those calculated by using SMD/M11L, which prompted us to use the combination of SMD/SCS‐MP2 to calculate the carbamate stabilities of substituted piperazines discussed in the Section 2.2. This superiority of SCS‐MP2 over M11L is in agreement with previous findings on alkanolamines 7c. The base strength of the amine (p K a ) influences the affinity of the amine to react with CO 2 ; the higher the p K a value of the amine, the faster the reaction rate with CO 2 21.…”

“…The decrease in carbamate stability of substituted piperazines somewhat parallels an increase in NC bond lengths of the carbamates of the substituted piperazines (see Table 3 and Figure S5 in the Supporting Information). The NC bond length in the carbamate of unsubstituted 1 is 1.471 Å, which is longer than the NC bond length in the carbamate of the industrially used solvent MEA (1.443 Å) 7c. In agreement with this, the carbamate stability (Δ G cs ) for 1 (4.13 kcal mol −1 ) is significantly lower than the Δ G cs of MEA, which is 5.83 kcal mol −1 .…”

“…Also, the calculations indicate that the carbamates formed at N b are more stable than those at N a . The carbamate stabilities of substituted piperazines can be correlated with the NPA charge on the nitrogen atom 7c. For example, the NPA charge on N a in 2 is −0.704, which results in a Gibbs free energy of 3.07 kcal mol −1 , whereas the NPA charge on N b is −0.710, which results in a Gibbs free energy of 4.64 kcal mol −1 , in line with this higher negative charge.…”

The Transition States for CO2 Capture by Substituted Ethanolamines

“…The highest NPA charge on the NH moiety was found for N ‐isopropylMEA ( 7 ), which is around −0.39 and followed by −0.38 and −0.37 for N ‐ethylMEA ( 6 ) and N ‐methylMEA ( 5 ), respectively. Furthermore, our previous studies showed that N ‐alkyl‐substituted MEAs form less stable carbamates than unsubstituted MEA 12b. This leads to an increase in the free amine concentration in solution, and thus to higher CO 2 absorption capacities.…”

Methanol formation by catalytic hydrogenation of CO2on a nitrogen doped zinc oxide surface: an evaluative study on the mechanistic pathway by density functional theory

“…63 Further investigation of the carbamate stability on the surface was done using ADMPMD calculations. This charge transfer from the surface reveals that the N doped zinc oxide is a more powerful Lewis base than pure zinc oxide.…”