Quantum-Mechanical and Thermodynamic Study of Amines and Ionic Liquids for CO2 Capture

Abstract

There is worldwide interest to develop improved CO2-capture solvents to help reduce cost of capture and environmental footprint. This thesis aims to contribute to this goal by studying computationally and experimentally a few aspects related to solvent development, considering both aqueous amines and ionic liquids. For pKa prediction of aliphatic amines, the performance of quantum-chemistry (QM) continuum-plus-correction methods was evaluated by comparison with the 1981 pencil-and-paper group-additivity method of Perrin, Dempsey, and Serjeant (PDS). The best continuum-plus-correction method has been achieved, and while it offers improvement over the original 1981 PDS method, it is inferior to a trivial update of the PDS method, and the latter is recommended for pKa prediction. Static QM calculations using continuum-plus-explicit-water models were used to examine the reaction mechanisms for CO2 capture by aqueous amines. For the first time, carbamate anions are correctly predicted by QM to be lower in energy than zwitterion or carbamic-acid forms. Zwitterions are most relevant at low amine concentrations (from single-amine versus two-amine modeling). Activation energies vary with pKa in a sufficient manner that the existence of the zwitterion may depend on pKa. QM-based ab initio molecular dynamic simulations of aqueous zwitterions were also performed, supporting the relevance of zwitterions at low concentrations, but also revealing 10-14- atom H+ transfer relays from zwitterion to transient carbamic acid forms. A database of Henry’s law constants for CO2 in 2701 ionic liquids at 25°C was prepared by computing predictions using the QM-based statistical-thermodynamic method COSMO-RS. The predictions agree well with experimental values, although the additionally computed predictions for selectivity and solvation enthalpy were not as good. A new polarity descriptor of ions and ionic liquids is introduced. Henry’s law constants are dissected into components to probe gas liquid interactions and compare the solubility of a gas in different ionic liquids. Based on the analysis, a number of ionic liquids are proposed for further experimental investigation, demonstrating the utility of COSMO-RS in screening of ionic liquids for CO2 capture. Based on the COSMO-RS study, three ionic liquids 1-alkyl-3-methyl imidazolium tris(pentafluoroethyl)-trifluorophosphate (alkyl = ethyl, butyl, and hexyl) were chosen for further experimental measurement of solubility of CO2 up to 2 MPa at temperatures of 10, 25 and 50°C using a gravimetric microbalance. The Henry’s law constants derived from experimental data compared favoribly with those predicted by COSMO-RS, and with previous experimental values for alkyl = ethyl and hexyl. Finally, the density, viscosity and excess molar enthalpy of the binary system {[bmim][Ac]+water} were experimentally determined at atmospheric pressure and at temperatures from 25 to 70°C. All the excess properties show strong negative deviation from ideality. Viscosity of pure [bmim][Ac] decreases significantly with addition of water and with increase in temperature. Excess enthalpies of equimolar binary mixtures with pure amines {(MEA, DEA, MDEA, TEA, AMP) + [bmim][Ac]} were less negative (less exothermic mixing) than the {[bmim][Ac] + water} system at 25°C.