Design and characterization of absorbent polymers for two-phase partitioning bioreactor applications
The production of fuels and chemicals from renewable biomass feedstocks can reduce our reliance on petroleum resources and help mitigate environmental issues associated with their use. However, many bioprocesses are limited by end-product inhibition, leading to lower product titer and reduced bioreactor productivity. Introducing a carefully selected solid absorbent polymer can reduce end-product inhibition by removing the product as it is formed and enriching it within the polymer phase. To be effective, the polymer must possess (1) a high affinity for the target solute, measured by its partition coefficient (PC), (2) a preference for solute absorption over water uptake, measured by selectivity (α), (3) biocompatibility towards the microorganism, and (4) sufficient physical strength to withstand the high shear bioreactor environment. The present work begins with an evaluation of thermodynamic activity models for a priori polymer phase selection. Accurate PC predictions were demonstrated for a range of target solutes in rubbery, amorphous materials. Further, polymer selection based on PC was shown to favor low molecular weight (MW) materials, whereas higher MW polymer yielded stronger, more solid-like absorbents. Compromises between absorptive and physical properties inherent to unimodal molecular weight distribution (MWD) polymers were mitigated by developing novel bimodal MWD formulations, made from mixing very low MW material (organic solvent or oligomer) with a high MW polymer. Solute absorption was also sensitive towards polymer crystallinity and water uptake; these factors were captured qualitatively by the thermodynamic models but with significantly reduced prediction accuracy. Remarkable improvements in both PC and α for n-butanol and n-octanol were realized by covalently tethering small amounts of imidazolium ionic functionality to a hydrophobic polymer, yielding an ionomer. Further improvements were achieved by preparing high ionic content polyelectrolytes, polymerized from reactive imidazolium ionic liquids (IL). The polyelectrolytes registered excellent PC values for several alcohols (n-butanol, iso-butanol, ethanol) as well as good solute/water selectivity. Importantly, both ionomer and hydrophobic polyelectrolyte formulations substantially eliminated cytotoxicity and water-solubility concerns associated with analogous ILs. These findings advance the state-of-the-art in absorbent materials for bioprocess separations and represent a promising avenue for future partitioning bioreactor research.
URI for this recordhttp://hdl.handle.net/1974/15380
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