Extraction by ionic liquids for the case of detoxification of lignocellulosic hydrolysates

This study deals with hydrophobic phosphonium ionic liquids (ILs), phosphinate and neodecanoate, used in liquid–liquid extraction for the purpose of complex detoxification of lignocellulosic hydrolysates from inhibitors, but preserving the sugar content. The topic is considered from two aspects, theoretical one in which extraction from a model multicomponent solution composed of acids, furan, phenolics, and sugars is investigated, and practically by employing a real rice straw hydrolysate. Using the model solution in cross-current extraction mode, the main process parameters, pH and concentration of the ILs, are studied. The extraction mechanisms of acids (sulfuric, gallic, acetic and levulinic acids) and aldehydes (vanillin and furfural) are established. Extraction of the acids in both ILs proceeds by a competitive mechanism until the two reactive H-bonding sites located at the two oxygen atoms in the IL’s anion are occupied. In addition to H-bonding, extraction of the phenolic acid is substantially assisted by hydrophobic interactions, while the sulfuric acid is readily extracted by protonation of the IL’s anion. An above-stoichiometric extraction of acids by phosphonium phosphinate has been found, which occurs by acid–acid H-bonds between phenolic and organic acids. Co-extraction between phenolic acid and phenolic and furanic aldehydes is observed which is based on the H-bonds that exist in acidic media and the staking interactions of the aromatic rings. The extraction of real rice straw hydrolysate carried out in three runs reaches a high removal of organic acids (over 63%), furans (over 80%) and phenolic compounds (over 97%) in each run.