Fractionation of Organic Fuel Precursors from Electrolytes with Membranes

Efficient membrane fractionation of small, neutral organics from electrolytes could improve the sustainability of biofuel production from microorganisms, but the separations achieved are often insufficient. To rationally develop improved membrane materials for these fractionations, we need to better understand the mechanisms that govern solute transport through different types of materials. To this end, we have studied the transport properties of a variety of commercially available (that belong to major classes of polymeric materials) and some newly synthesized membranes. We determined that membranes belonging to a particular class have signature transport properties—for example, fully aromatic polyamides (FA-PA) are slightly more permeable to glycerol than NaCl, while the converse is true for semiaromatic polyamides (SA-PA)—although all glycerol/NaCl separation factors are near unity. Selected membranes were further probed with different reduced-carbon/electrolyte combinations. The cellulose–acetate (CA) membrane achieved the greatest separation between ethanol and electrolyte (NaCl, LiCl, Na 2 SO 4 ), and the SA-PA membranes are better at fractionating larger reduced-carbons (glucose and sucrose) from the monovalent electrolytes. Meanwhile, an order-of-magnitude improvement in separation factor was found for the challenging glycerol/electrolyte fractionation with the SA-PA membranes when a divalent anion is used. The main transport mechanisms are interpreted based on Donnan exclusion and thermally activated transport through the polymer. The CA and FA-PA membranes appear to separate electrolytes through a predominantly steric-based mechanism, while the SA-PA membranes are more sensitive to anion valency. Furthermore, while solute size clearly plays a role in determining neutral solute transport, the relative roles of solubility (i.e., polymer-permeant interactions) versus polymer free-volume in the transport of small organics through the different materials remain unclear.