Molecular Design Strategies to Reduce the Viscosity of Non-Aqueous Carbon Capture Solvents

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.