In order to replace 30% of the petroleum transportation fuels with biofuels, a large scale biorefinery industry needs to be established. A crucial component could be the development of cost effective separation methods that improve process economics. Membrane extraction has been developed on a small scale and showed good removal of acetic acid, sulfuric acid, furfural and HMF from dilute acid pretreated biomass hydrolysates. These compounds are inhibitors for the down-stream fermentative microorganisms. Carboxylic acids and other toxic compounds have been successfully extracted into an organic phase consisting of octanol and Alamine 336, a tertiary amine, containing aliphatic chains of 8-10 carbon atoms.

We have conducted molecular modeling to understand the fundamental mechanisms behind this extraction process using two extraction solvents. We determined free energy changes for extracting the carboxylic acids, furfural and HMF from the hydrolysates to octanol and ethanol with and without the presence of Alamine 336 using the Gaussian 03 package. Extraction free energy was determined using a thermodynamics cycle based on the gas phase and solvation free energies of the two phases. Density functional theory (DFT) based hybrid B3LYP method has been used to calculate the gas phase free energies. Solvation free energies were calculated using the PCM model. The formation of the carboxylic acid dimer is found to enhance the extraction partition coefficient. Additional validation of the theoretical results was conducted through shaking and membrane extraction experiments.