Influence of the selective imine formation with 2-octanone on the liquid–liquid extraction of hexamethylenediamine from water

• Formation of hydrophobic imines from HMDA and 2-octanone proven via Raman • Formation of imines increases extraction efficiency of primary diamine HMDA • Increased extraction efficiency of 2-octanone/1-octanol mixture over pure solvents • Selective increase of extraction efficiency of primary amines with 2-octanone The separation of amines from aqueous streams is necessary in many processes, including the chemical recycling of polymers. Liquid–liquid extraction is a promising method for the separation of amines from aqueous environment, but poses a challenge in terms of selectivity between degrees of substitution. To increase the selectivity towards primary amines, we propose the use of ketones as extractants, as they can form hydrophobic imines with primary amines, increasing the extraction into the organic phase. To investigate using ketones in liquid–liquid extraction, we employed 2-octanone as extractant, 1-octanol as unreactive diluent, hexamethylenediamine (HMDA) as primary amine, N,N’-bis-(hydroxyethyl)ethylenediamine and N,N’-dimethyl-hexamethylenediamine as hydrophilic and hydrophobic secondary amines, respectively, and N,N,N’,N’-tetrakis-(hydroxyethyl)ethylenediamine and N,N,N’,N’-tetramethyl-hexamethylenediamine as hydrophilic and hydrophobic tertiary amines, respectively. The formation of imines from 2-octanone and HMDA was confirmed through Raman spectroscopy. The extraction efficiency of HMDA was increased from (58.7(05))% with pure 1-octanol to (82.7(03))% with a mixture with an initial mass fraction of 66.3% 2-octanone and 33.7% 1-octanol. NaOH addition was shown to increase the extraction efficiency up to (92.0(02))%, while H 2 SO 4 addition decreased the extraction efficiency. The extraction efficiency of higher-substituted amines was observed to be either unaffected or decrease with increasing 2-octanone concentration. As 2-octanone increases the extraction efficiency of the primary amine HMDA selectively, ketones could be a promising extractant for amines in future recycling processes.