Aaron D. Wilson

Concentration dependent speciation and mass transport properties of switchable polarity solvents

Tertiary amine switchable polarity solvents (SPS) consisting of predominantly water, tertiary amine, and tertiary ammonium and bicarbonate ions were produced at various concentrations for three different amines: N,N-dimethylcyclohexylamine, N,N-dimethyloctylamine, and 1-cyclohexylpiperidine. These amines exhibit either osmotic or non-osmotic character as observed through forward osmosis, which led to this study to better understand speciation and its influence on water transport through a semi-permeable membrane. For all concentrations, several physical properties were measured including viscosity, molecular diffusion coefficients, freezing point depression, and density. Based on these measurements, a variation on the Mark–Houwink equation was developed to predict the viscosity of any tertiary amine SPS as a function of concentration using the amines molecular mass. The physical properties of osmotic SPS, which are identified as having an amine to carbonic acid salt ratio of ∼1 : 1, have consistent concentration dependence behavior over a wide range of concentrations, which suggests osmotic pressures based on low concentrations freezing point studies can be extrapolated reliably to higher concentrations. The observed physical properties also allowed the identification of solution state speciation of non-osmotic SPS, where the amine to carbonic acid salts ratio is significantly greater than one. These results indicate that, at most concentrations, the stoichiometric excess of amine is involved in solvating a proton with two amines.

Density functional theory analysis of the impact of steric interaction on the function of switchable polarity solvents

A density functional theory (DFT) analysis has been performed to explore the impact of steric interactions on the function of switchable polarity solvents (SPS) and their implications on a quantitative structure-activity relationship (QSAR) model previously proposed for SPS. An X-ray crystal structure of the N,N-dimethylcyclohexylammonium bicarbonate (Hdmcha) salt has been solved as an asymmetric unit containing two cation/anion pairs, with a hydrogen bonding interaction observed between the bicarbonate anions, as well as between the cation and anion in each pair. DFT calculations provide an optimized structure of Hdmcha that closely resembles experimental data and reproduces the cation/anion interaction with the inclusion of a dielectric field. Relaxed potential energy surface (PES) scans have been performed on Hdmcha-based computational model compounds, differing in the size of functional group bonded to the nitrogen center, to assess the steric impact of the group on the relative energy and structural properties of the compound. Results suggest that both the length and amount of branching associated with the substituent impact the energetic limitations on rotation of the group along the N-R bond and NC-R bond, and disrupt the energy minimized position of the hydrogen bonded bicarbonate group. The largest interaction resulted from functional groups that featured five bonds between the ammonium proton and a proton on a functional group with the freedom of rotation to form a pseudo six membered ring which included both protons.

Electrochemical production of syngas from CO2 captured in switchable polarity solvents

An operational advantage that enables the deployment of technologies for the valorization of CO2 can be achieved through the integration of the capture and conversion technologies. In that perspective, switchable polarity solvents (SPS) were assessed as re-usable capture-electrolyte media for the electrochemical production of syngas at low temperatures and pressures. A polymer electrolyte membrane cell was used to liberate captured CO2 gas in proximity to the cathode without the addition of CO2 gas. Due to the proximate location of release, the produced syngas had minimal CO2 dilution with H2 : CO ratios between 2 to 4. For the first time captured CO2 has been reduced to CO with conversions and yields over 70% and at current densities over 100 mA cm−2.