(71d) Molecular Design Strategies to Reduce the Viscosity of Non-Aqueous Carbon Capture Solvents

Authors: 
Cantu, D. C., Pacific Northwest National Laboratory
Malhotra, D., Pacific Northwest National Laboratory
Koech, P. K., Pacific Northwest National Laboratory
Heldebrant, D. J., Pacific Northwest National Laboratory
Rousseau, R., Pacific Northwest National Laboratory
Glezakou, V. A., Pacific Northwest National Laboratory
In power generation, high viscosities encumber the use of post-combustion non-aqueous CO2 capture solvents. Using single molecule CO2 binding organic liquids (CO2BOLs) as example, the key molecular features that determine viscosity and CO2 uptake kinetics are identified. The close proximity of the alcohol and amine sites involved in CO2 binding result in the concerted formation of a Zwitterion containing both an alkyl-carbonate and a protonated amine. Capture is controlled by viscosity since the small binding energy barrier suggests that diffusion is rate limiting. The viscosity of CO2BOLs increases exponentially as a function of CO2 loading. Inter-molecular hydrogen bonding between positively and negatively charged functional groups ultimately determines viscosity. The hydrogen-bonding network can be controlled by chemically tuning the solvents to favor an internal hydrogen bonding structure. Additionally, a molecular design strategy to significantly reduce viscosity by shifting the proton transfer equilibrium toward a non-charged acid/amine species, as opposed to the ubiquitously accepted zwitterionic state, is presented. Based on knowledge obtained from molecular simulation, a reduced model was developed that predicts CO2BOL viscosity given a few structural parameters and CO2 loading. In collaboration with experiment, candidate compounds were synthesized to measure their viscosities and verify the model.