Improvement of Biofuel Fermentation Processes With Use of Poly(vinyldodecylimidazolium bromide) for In-situ Product Removal
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The production of biofuels through fermentation has become increasingly important due to the need for sustainable energy sources. The fermentation processes of ethanol and n-butanol, by Saccharomyces cerevisiae and Clostridium acetobutylicum, respectively, suffer from end-product inhibition, hindering final product concentrations, volumetric productivity, and product recovery. To alleviate these limitations, in-situ product removal (ISPR) methods can be implemented, via two-phase partitioning bioreactor (TPPB) technology. This work used a polyelectrolyte absorbent phase, poly(vinyldodecylimidazolium bromide) [P(VC12ImBr)], in TPPB systems for the removal of the target molecules, ethanol and n-butanol, with focus on the latter due to more severe product inhibition and its greater promise as a biofuel. P(VC12ImBr) was synthesized and characterized for ISPR implementation and was found to have a partition coefficient (PC) and selectivity of 1.1 and 7.2 for ethanol, and a PC of 6.5 and selectivity of 46 for n-butanol. Characteristics of P(VC12ImBr) were found to be favorable for TPPB implementation, including its density, diffusivity for n-butanol, and complete biocompatibility with S. cerevisiae and C. acetobutylicum. A differential scanning calorimetry (DSC) scan of P(VC12ImBr) found its melting temperature to be -17°C and a glass transition temperature above 200°C, which can be reduced by plasticization with water. Anion exchange between bromide in P(VC12ImBr) and sulphate and phosphate ions in fermentation medium was observed, however this did not affect its PC or selectivity. When implemented in ethanol and acetone-butanol-ethanol (ABE) ISPR fermentations, P(VC12ImBr) absorbed target molecules from the fermentation broth, however improvements relative to a control were observed only for the ABE fermentation with the mass fraction of sorbent used. In the ABE ISPR fermentation there was greater substrate conversion and an increase in volumetric productivity of 76%. The water + n-butanol + P(VC12ImBr) ternary system confirmed P(VC12ImBr)’s preference for n-butanol by concentrating dilute solutions above its aqueous solubility limit. To examine n-butanol absorbed by P(VC12ImBr), a 1.5 wt% n-butanol solution was concentrated to 25 wt% n-butanol, which was thermally removed and condensed to show phase separation. The ability of P(VC12ImBr) to concentrate dilute solutions of n-butanol above its solubility limit would allow for an improved n-butanol purification process.