Reactive liquid–liquid extraction of bio-based 3-hydroxypropionic acid using a biocompatible organic phase containing a tertiary amine: A model-based approach to elucidate dominant mechanisms

Abstract

This work studies reactive liquid–liquid extraction of 3-hydroxypropionic acid (3-HP) from aqueous solutions using a biocompatible organic phase consisting in N,N-didodecylmethylamine (DDMA) (20% v/v) diluted in 1-dodecanol (40% v/v) and dodecane (40% v/v). The objective was to propose an equilibrium model based on the law of mass action accounting for the main phenomena occurring in the system. A set of equilibrium extraction experiments was performed in the initial acid concentration range 0.0028 – 1 mol L-1 in order to determine the extraction yield, equilibrium pH and to collect infrared spectra of the loaded organic phases. FT-IR spectra allowed to identify and elucidate the main mechanisms of acid–amine interaction, which were taken into account in the model. The formation of 1:1 acid–amine stoichiometry complex through ion-pairing was determined as the dominant mechanism in all the concentration range. For initial acid concentrations above 0.11 mol L-1, the formation of 2:1 acid–amine complexes through H-bonding of acid molecules with 1:1 complexes was also observed and determined to be almost as important as ion-pair formation at 1 mol L-1. The presence of impurities in the organic phase was taken into account by introducing a side reaction essential to represent low extraction yields and high equilibrium pH values observed at low initial acid concentrations (< 0.02 mol L−1). A global sensitivity analysis of the model’s parameters pointed out three concentration regions based on the relative contribution of each phenomenon: (i) for initial acid concentrations below 0.02 mol L−1, the main phenomena are 1:1 ion-pair formation and the side reaction; (ii) between 0.02 mol L−1 and 0.11 mol L−1, mainly 1:1 ion-pair formation; and (iii), for concentrations above 0.11 mol L−1 both considered complexation reactions i.e., 1:1 ion-pair formation and 2:1 H-bonding. The proposed model allowed thus to evaluate the relative importance of the phenomena occurring in the system in all the studied acid concentration range.