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 surveyed the transport properties of a variety of commercially- 

available and newly-synthesized membranes that belong to major classes of polymeric materials. 

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 semi-aromatic 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 

achieves the greatest separation between ethanol and electrolyte (NaCl, LiCl, Na2SO4), 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 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 precise role of solubility (i.e. polymer- 

permeant interactions) versus polymer free-volume in the transport of small organics through the 

different materials remains unclear.