ABSTRACT
Substituted amines are a popular choice as molecules to selectively react with and separate CO(2) from gas mixtures. Such separations are of particular interest, for example, for CO(2) separations for carbon capture and sequestration. It is desirable to tune amine-CO(2) reaction energies to suit a particular separation. Herein, we use DFT-B3LYP simulations to characterize the products and energetics of reactions of CO(2) with a range of substituted amines, considering both 1:1 and 2:1 amine/CO(2) reaction stoichiometries. The results show that by adjusting both the nature and the placement of functional groups, it is possible to tune reaction energies over a substantial range. Decomposition of the 2:1 reaction into separate carbamate and ammonium formation steps shows that the Brønsted basicity and Lewis basicity towards CO(2) are largely uncorrelated and provide an independent means of tuning the overall reaction energies.
Subject(s)
Amines/chemistry , Carbon Dioxide/chemistry , Quantum Theory , Ammonia/chemistry , Carbamates/chemistry , Carbon Dioxide/isolation & purification , Electron Transport , Models, Molecular , Molecular Conformation , Protons , ThermodynamicsABSTRACT
Amino acid ionic liquid trihexyl(tetradecyl)phosphonium methioninate [P(66614)][Met] and prolinate [P(66614)][Pro] absorb CO(2) in nearly 1:1 stoichiometry, surpassing by up to a factor of 2 the CO(2) capture efficiency of previously reported ionic liquid and aqueous amine absorbants for CO(2). Room temperature isotherms are obtained by barometric measurements in an accurately calibrated stirred cell, and the product identity is confirmed using in situ IR. Density functional theory (DFT) calculations support the 1:1 reaction stoichiometry and predict reaction enthalpies in good agreement with calorimetric measurements and isotherms.
