Imidazolium-Type Anion Exchange Membranes for Improved Organic Acid Transport and Permselectivity in Electrodialysis

Abstract

Most commercial anion exchange membranes (AEMs) deploy quaternary ammonium moieties. Alternative cation moieties have been explored in AEMs for fuel cells, but there are no studies focused on examining alternative tethered cations in AEMs for ionic separations – such as organic acid anion transport via electrodialysis. H-cell and conductivity experiments demonstrate that tethered benzyl 1-methyl imidazolium groups in polysulfone AEMs enhance lactate conductivity by 49% and improved lactate anion flux by 24x when compared to a quaternary benzyl ammonium polysulfone AEM. An electrodialysis demonstration with the imidazolium-type AEM showed a 2x improvement in lactate anion flux and 20% improvement in permselectivity when benchmarked against the quaternary ammonium AEM. Molecular dynamics and 2D NOESY NMR revealed closer binding of lactate anions to the imidazolium cations when compared to the quaternary ammonium cation. It is posited that this closer binding is responsible to greater flux values observed with imidazolium-type AEM.

Integrated Ion-Exchange Membrane Resin Wafer Assemblies for Aromatic Organic Acid Separations Using Electrodeionization

“…Because the equilibrium partitioning coefficient difference for p-coumarate between QIPSf and QAPSf is not commensurate with the permeability coefficient differences, we inferred that the p-coumarate diffusivity was greater in QIPSf over QAPSf as permeability is the product of the partitioning coefficient and the diffusion coefficient. This observation was also observed in our previous work 27 for a lactate (i.e., QIPSf AEMs promoted lactate transport when compared to QAPSf AEMs). Because QIPSf enhances pcoumarate permeability, we used this material as a AEM and resin wafer binder in the new MWAs.…”

“…The previous static studies of QIPSF and QAPSF with lactate anions showed that lactate anions bind closer to the imidazolium cations when compared to the quaternary ammonium cation. 27 Similar observations are seen with the p-coumarate anion when considering the same interaction sites (labeled carbons in Figure 8) of the imidazolium group and ammonium groups. The radial distribution function, g(r), calculated between the labeled carbon in each head group in each case with the carbonyl carbon of p-coumarate (Figure 8) shows that p-coumarate association with imidazolium head groups is prominent due to the larger peak height for pcoumarate−imidazolium compared to p-coumarate−ammonium.…”

“…Fabrication of Ionomer Resin Wafer Used in MWAs. Quaternary benzyl n-methyl imidazolium chloride poly(arylene ether sulfone) (QIPSf) was used as an AEM, and the RW ionomer binder in this study because our previous work 27 showed that the imidazolium groups foster greater organic acid anion transport when compared to the conventional quaternary ammonium groups. The RWs with ionomer binders were fabricated by mixing a 14 wt % solution of QIPSf in n-methylpyrrolidine (NMP) with a ratio of anion and cation exchange resins (AERs and CERs) of 1:1.05, sodium chloride, and water (ionomer binder:resins:salt:water in a 2.4:2:1:0.5 ratio).…”

“…The ionomer binder and AEM in the MWA feature tethered quaternary benzyl 1-methyl imidazolium groups to promote organic acid transport. 27 The laminated membranes to the resin wafer yield a 38% reduction in interfacial transport resistance. To realize the MWA in an EDI stack, a new gasket design was devised to mitigate leaking issues.…”

“…This figure also shows the bulk material ohmic resistances as well as the ohmic interfacial resistances. The ionomer binder and AEM in the MWA feature tethered quaternary benzyl 1-methyl imidazolium groups to promote organic acid transport . The laminated membranes to the resin wafer yield a 38% reduction in interfacial transport resistance.…”