Bio-oils produced by biomass upgrading processes, including fast pyrolysis, contain valuable chemicals that can be processed into value-added chemical feedstocks and fuels. However, separating and refining these oils remains challenging, as conventional thermal methods often induce polymerization or oxidation of the bio-oil compounds. Membrane-based separations can potentially address this problem by avoiding high temperature operation. Unfortunately, most membrane materials and processes struggle to recover dilute compounds from complex mixtures as well as separate solutes from other solutes. Here, we use a crossflow permeation system with a novel spirocyclic polytriazole membrane (DUCKY-9) to separate a synthetic bio-oil mixture of guaiacol, water, methanol, acetol, and glucose via pressure-driven liquid phase permeation. Our initial hypothesis was that the larger guaiacol and glucose compounds would be partially rejected by the DUCKY-9 thin film composite (TFC) membrane based on the molecular weight cut-off of the DUCKY-9 membrane. Remarkably, the membrane selectively permeates guaiacol—even though it is highly dilute and the second-largest molecule. We find that guaiacol’s flux is strongly coupled with methanol’s flux, enabling “uphill” permeation of guaiacol in terms of membrane driving forces without second law violations. We hypothesize that this phenomenon stems from solubility and chemical affinity between the polymer and solvents, and we demonstrate that tuning this flux coupling can dramatically enhance separation performance. Notably, increasing the feed’s methanol content by only 5 mol% boosts guaiacol’s selectivity over glucose from 10 to 25, despite their similar sizes (i.e. 124 Da vs 180 Da). Our experiments suggest that this uphill membrane permeation process may enable recovery of diluted and valuable compounds from complex mixtures.
