(313b) Tuning Electrostatic Interactions for Selective Electrochemical Organic Acid Separations

Organic acids are weakly ionizable species that are susceptible for purification from process streams using electrochemical processes. Recently the fermentation route has grown attention as a means of producing value-added chemicals, such as organic acids, from a renewable feedstock and aiding the circular economy. The recovery of organic acids from fermentation streams such as lactic acid (in polylactide production) represents the majority cost component in these biochemical production processes. Therefore, a material advancement that would target the removal of these organic acid molecules would significantly reduce the overall production cost of certain bioderived products.

In this talk, I will discuss our recent findings that resonance stabilized cation groups, such as imidazolium groups, favorably interact with carboxylic acid groups found in organic acids which can be tuned to accelerate the targeted recovery of organic acids from process streams. Our approach was inspired by the CO₂ reduction community where it has been shown that imidazolium-functionalized polymers promote electrochemical CO₂ reduction. It is hypothesized that carbene sites on the carbon atoms in the imidazolium ring can be generated and these carbenes have a chemical affinity towards the carboxyl groups in organic acids leading to selective removal over other types of anions and enhanced transport rates.

The cation head group chemistry of imidazolium functionalized AEM (QIPSf) was benchmarked against a quaternary cyclic ammonium functionalized AEM (QAPSf). The quaternary cyclic ammonium groups cannot form carbenes. Conductivity measurements of QIPSf membranes demonstrated favorable conductivity properties versus QAPSf membranes in aqueous organic acid solutions. The equilibrium uptake of lactic acid by the two membrane chemistries was evaluated and did not indicate a significant difference. Further equilibrium ionic flux experiments in an H-cell setup indicated greater lactate crossover rates with QIPSf membranes. These findings suggest that the ion diffusion rate of lactate through imidazolium-functionalized membranes are the dominant property that affect lactate permeability. Lastly the selective properties of QIPSf were compared with QAPSf with a mixture of common fermentation broth contaminant molecules (chloride, sulfates and phosphates). Overall, this research demonstrates that resonance stabilized cation groups, such as imidazolium, can be capitalized upon to accelerate the selective removal of organic acid containing molecules and improve the cost metrics and energy efficiency for a plethora of separation applications such as purifying bio-oils, recovery of organic acids for feed monomers in the production of bioplastics (and plastic upcycling), and specialty chemicals such as ferulic acid.