The unusual transport of phenolic compounds through poly(triazole) membranes

Bio-oils are the product of various biomass upgrading processes such as fast pyrolysis and reductive catalytic fractionation and contain valuable chemicals such as phenolics. However, these mixtures are difficult to refine because many components undergo thermal degradation, polymerization, or phase instability when heated, limiting the use of conventional distillation. We compared a spirocyclic polytriazole (DUCKY-9) membrane against two commercial nanofiltration membranes in the separation of simple and synthetic bio-oil mixtures via pressure-driven permeation. DUCKY-9’s aromatic, highly sorptive polymer matrix strongly interacts with phenolics, enabling solute-solvent coupling not observed in the commercial membranes. As a result, DUCKY-9 enriched guaiacol in the permeate (6.2-11 mol%, averaging 7.7 mol % from a 5 mol % feed) while strongly retaining glucose, whereas the commercial membranes showed only conventional size-based behavior and did not enrich guaiacol. The transport of guaiacol through the membrane was found to be coupled with the flux of methanol, and complementary sorption measurements indicate that methanol is the primary “sorpvecting” agent responsible for the permeation of phenolic compounds “uphill” against the concentration gradient. With the addition of 5 mol % methanol, the guaiacol/glucose selectivity increased by 2.6 relative to performance without methanol, resulting in a guaiacol/glucose separation factor of 13.8. Because this uphill permeation concentrates dilute solutes, it may also improve solute-concentrating membrane cascade processes such as osmotically assisted reverse osmosis, reducing the membrane area required for separation by about 4.3-fold. • Phenolics permeate “uphill” through spirocyclic DUCKY-9 polymer membranes • Methanol flux sweeps phenolics, enriching guaiacol on the permeate side • Sorption studies show solvent-coupling plays a key role in phenolic transport mechanism • Solvent-coupled transport boosts guaiacol-to-glucose separation factor by 2.5-fold • Solvent-coupled pretreatment could substantially reduce membrane area needed for solute recovery system