(169h) Computational and Experimental Studies of Hydrophoic, Nonaqueous, Nonvolatile, Low Viscous, and High CO2 Absorption Chemical Solvents

Solvents with high CO_­2 loadings in the intermediate CO₂ pressure range of 2-5 bar have lots of industrial applications, such as CO₂ capture from the steam methane reforming process. Recently, experimentalists at NETL have synthesized a new class of solvent, which contains NH and COO or NCO groups. This class of solvent was found to give high CO₂ loadings at the pressure range of 2-5 bar due to a reversible chemical reaction at the amine site with CO₂. In this presentation, we present the computational and experimental studies of this class of solvent.

Due to the complicated molecular structure for the EMA-NH-EMA solvent molecule, a python script code was developed to automatically generate 30-40 different initial structures, which were optimized by using the UFF classical force field, for both the reactants and products by using the RDKit package. These optimized initial configurations were then further optimized at the B3LYP/6-311++g(d,p) level of theory in the gas phase by using the Gaussian 09 software package. Simple scripts were developed to automate to conduct the quantum mechanical (QM) calculations and analyze the QM results obtained from Gaussian calculations. In addition, classical force field was developed for the solvent to predict its physical properties, such as vapor pressure obtained from the solvation free energy calculation by using the BAR method implemented in Gromacs, solvent viscosity, and CO₂ diffusivity, CO₂ and H₂ physical solubilities. QM calculations show that for the reactant EMA-NH-EMA, a configuration with intra-hydrogen bonding between H (NH) and O of (CO) exhibits the lowest energy, while an extended configuration without intra-hydrogen bonding network exhibits about 9 kJ/mol higher energy. For CO₂ reaction with EMA-NH-EMA to form carbamic acid (NCOOH), two typical configurations are possible. The first configuration is that H atom of NCOOH forms intra-hydrogen bond interaction with O of CO, and the second one is the extended configuration with NCOOH pointing outwards without intra-hydrogen bonding. In contrast to the reactant, for the reaction product, the second extended configuration is more stable and exhibits about 10 kJ/mol less energy than the first configuration. Molecular dynamics (MD) simulations using the second extended configuration for the product shows a high viscosity of 62.3 cP compared with the low viscosity of 7.8 cP for the neat solvent. The high viscosity of 62.3 cP was due to the inter-hydrogen bonding, which leads to CO₂ diffusivity to decrease by 3.4 times in the product compared with the neat reactant. In contrast, the experimental data does not suggest a significant viscosity increase upon CO₂ reaction with the EMA-NH-EMA solvent. This inconsistency for the product viscosity between simulations and experimental data suggests that the first intra-hydrogen bond configuration for the product will be present and simulations will be presented. Computations also suggest that the EMA-NH-EMA solvent exhibits a very low vapor pressure of 0.056 (8) Pa at 298 K, which is comparable to the nonvolatile Selexol solvent. Finally, CO₂ physical solubilities in both the neat solvent and the product have been calculated and they were found to be reasonably high (1.21-1.41 mol/L.MPa at 298 K), which is consistent with the experimental observation that CO₂ solubility still appreciably increases at elevated pressures instead of approaching a plateau region for a typical chemical solvent. H₂ physical solubilities in both the neat solvent and the product were also calculated and they were found to be favorably low (0.0199 - 0.02484 mol/L.MPa at 298 K) and they will be compared with experimental data. Temperature effects on CO₂ and H₂ physical solubilities will also be shown. Finally, we note that due to significant amounts of CH₂ groups in the solvent, this class of solvent is very hydrophobic.