CO2 Solubility in Organophosphate Physical Solvents Wherein Alkyl Groups Are Replaced with Poly(Ethylene Glycol) Groups

Previous success in improving the CO2 capacity of physical solvents for pre-combustion carbon capture by imparting poly(ethylene glycol) (PEG) functionality led us to compare tributyl phosphate (TBP), tri-isobutyl phosphate (TiBP) and three analogous organophosphate solvents in which the length of PEG-substitution was varied. The PEG-substituted solvents proved to have acceptable densities and viscosities for the application of interest, but all three solvents showed poorer CO2 absorption than TBP or TiBP. Inclusion of hydrophilic PEG groups in solvents (1) – (3) also led to the undesired absorption of larger amounts of water from humidified N2 compared to TBP and TiBP. Computational studies of the analogous organophosphate solvents revealed that all solvents had the lowest partial negative charges, closest CO2 interaction, and largest CO2 interaction energy at the double bonded phosphoryl O atom. The fractional free volumes were computed and was found to be largest for TiBP and grew progressively smaller as the length of the PEG group grew longer in solvents (1) – (3). Although introducing PEG groups to these molecules increased the number of interaction sites with CO2, solvents (1) – (3) showed poorer CO2 absorption than TBP and TiBP due to their decreased solvent fractional free volume.