title: Improving the capture of CO2 by substituted monoethanolamines: electronic effects of fluorine and methyl substituents.
abstract: The influence of electronic and steric effects on the reaction between CO(2) and monoethanolamine (MEA) absorbents is investigated using computational methods. The pK(a) of the alkanolamine, the reaction enthalpy for carbamate formation, and the hydrolytic carbamate stability are important factors for the efficiency of CO(2) capture. The steric and electronic effects of CH(3), CH(2)F, CHF(2), CF(3), F, dimethyl, difluoro, and bis(2-trifluoromethyl) substituents at the α carbon of MEA on this reaction are investigated. Density functional theory (DFT) (B3LYP, M06-2X, M08-HX and M11-L) and ab initio methods [spin component-scaled second-order Møller-Plesset theory (SCS-MP2), G3], each coupled with solvent models [conductor-like polarizable continuum model (CPCM) and universal solvation models (SM8 and SMD)], are shown to yield accurately calculated pK(a) values of the substituted MEAs. Specifically, G3, SCS-MP2, and M11-L methods coupled with the SMD and SM8 solvation models perform well with a mean unsigned error (MUE) of only 0.15, 0.24 and 0.25 pK(a) units, respectively. SCS-MP2 is used to calculate the reaction enthalpy for carbamate formation and the carbamate stability towards hydrolysis. With the introduction of β-fluoro substituents (especially the CH(2) F moiety) the reaction enthalpy for the formation of carbamates can be fine-tuned to be less exothermic than that using the unsubstituted MEA. This implies a reduced energy requirement for the solvent-regeneration step in the post-combustion carbon-capture method, which is currently the energy-limiting step in efficient CO(2) capture. β-Fluoro-substituted MEAs are also shown to form less stable carbamates than MEA. Thus, β-fluoro-substituted MEAs display a great potential for the use in the post-combustion carbon-capture process. Finally, a clear correlation is observed between the gas-phase basicity and the tendency to form carbamates. This allows for the rapid prediction of which species will be formed experimentally, and thus the CO(2)-absorbing capacities of alkanolamines can be estimated.
authors.affiliation: Laboratory of Organic Chemistry, Wageningen University, Wageningen, The Netherlands.
authors.forename: Satesh
authors.initials: S
authors.lastname: Gangarapu
authors.forename: Antonius T M
authors.initials: AT
authors.lastname: Marcelis
authors.forename: Han
authors.initials: H
authors.lastname: Zuilhof