A major obstacle to the large scale industrial use of biobased products and biofuels is the lack of efficient, cost-effective separation methods. Separations operations currently account for 60-80% of the processing costs of most mature chemical processes.  Here we focus on the development of membrane extraction as a low cost, robust separation process in future biorefineries.  As membrane extraction is non-dispersive it overcomes all of the disadvantages of conventional extraction.

Acetic acid is produced during thermochemical pretreatment of lignocellulosic biomass. It is a weak acid that is strongly inhibitory to microorganisms used for bioconversion of sugars.  Removal of acetic acid could be essential for increasing ethanol yields during fermentation.   

We have conducted experiments using dilute sulfuric acid pretreated corn stover.  Acetic acid, in its protonated form, was extracted into an organic phase consisting of octanol and Alamine 336, a tertiary amine, containing 8-10 carbon aliphatic chains.  Importantly, acetic acid removal is most efficient at pH values below 4.8, the pKa of acetic acid, thus no pH adjustment is required after pretreatment.  Further as sulfuric acid is co-extracted the pH of the hydrolysate increases during extraction.  Our results indicate co-extraction of furfural, hydroxymethylfurfural, acid soluble lignin and other phenolic compounds.  Thus addition of membrane extraction to remove acetic acid may simplify and/or eliminate current hydrolysate detoxification technologies such as overliming.  Development of a practical membrane extraction process for removal of weak acids such as acetic acid depends on carefully choosing the organic diluent and extractant (octanol and Alamine 336 used here).