Introduction
To reduce the usage of fossil-based resources, the transition to a circular economy is necessary. To implement a circular economy, depolymerization of plastic waste is an important link in this economy by recovering raw materials for new polymers.
Polymers such as polyurethanes and polyamides can be depolymerized in water under suitable conditions [1, 2]. The hydrolysis mixture of polyurethane contains amines and polyols, while polyamide is hydrolysed to amines and carboxylic acids. Additives, which are added in the production, make up approx. 20 % of the total volume of plastic products marketed and are also found in the mixture after depolymerization[3, 4]. Thus, separation techniques that are selective towards the valuable target monomers are crucial for the downstream development of depolymerization processes.
Selective extraction of amines from water can be achieved, by e.g. pH-controlled physical extraction [5] or carboxylic acid-mediated reactive extraction [6]. Both methods utilize the basic properties of the amines to achieve selectivity. As additives and monomers can also contain amines [7, 8], a mechanism targeting primary amines as the polymer-forming functional group is desirable. Krzyżaniak et al. (2013) utilized the reversible formation of a hydrophobic imine from a primary amines and a hydrophobic aldehyde for reactive extraction of 1,4-butylenediamine. Therefore, our goal is to evaluate the feasibility of a pH-controlled liquid-liquid extraction in the purification process of amines found in the depolymerized plastic mixtures and examine the application of imine formation to enhance the selectivity of the amine extraction.
Experimental
To evaluate the feasibility of pH-controlled liquid-liquid extraction and imine formation for recovering amines from depolymerized mixtures, two use cases of depolymerised plastic mixtures were investigated.
The first case was polyurethane foam, where both model mixtures and depolymerised mixtures of polyurethane foam, containing the target components polymeric methylenedianiline (pMDA) and polyols were investigated. Ethyl acetate or
dichloromethane were used as organic solvents, the pH was controlled via hydrochloric or sulfuric acid. The solvents were evaporated, and the residue was weighed to determine the amount of transition component extracted. NMR spectroscopy was used with the model mixture to determine the polyol and pMDA content.
Additionally, the application of the imine formation to improve the extraction of amines found in depolymerized mixed waste fractions was explored using a model system. 1,6-Hexanediamine, which is a precursor for nylon 66, and ethoxylated derivatives of ethylenediamine as a model for amine-based polyols were chosen as model compounds. Extraction equilibria were measured with 1-octanol as diluent, 2-octanone as molecular separation agent that can form imines, and aqueous solutions with sodium hydroxide or sulfuric acid. Sodium hydroxide and sulfuric acid affect the pH-value and thus the dissociation degree of the amines. NMR spectroscopy was used to determine the concentration of amine compounds before and after extraction. Raman spectroscopy was employed to validate the formation of imines
Results
The extraction experiments with the model mixtures of pMDA and polyols showed a necessity of large solvent amounts to achieve pure polyol and pMDA fractions in polyurethane-based depolymerized mixtures. This is in part due to amine-started polyols, which exhibit different distribution coefficients depending on the pH, similar to the aromatic amines. As pH-control is not sufficient for selective amine extraction, the imine formation was investigated as extraction method.
For amines found in depolymerized mixed waste fractions, the experiments with the model system showed that the extraction efficiency of 1,6-hexanediamine at natural pH was increased from 58.8±0.8 % with no 2-octanone in the organic phase to 84.6±0.6 % with 33 wt% 2-octanone in the organic phase. Raman-spectra confirmed the formation of imines in the organic phase. during extraction at basic pH level. The formation only occurred when both 2-octanone and unprotonated 1,6-hexanediamine were present. The extraction of secondary and tertiary amines was not affected by the addition of the molecular separation agent. The approach could be used to selectively extract primary amines from higher substituted amines.
Conclusion
Our work focused on the selective extraction of amines found in depolymerized mixtures. We showed that the selectivity of the pH-controlled extraction of primary amines poses a challenge, which can be alleviated by utilizing imine formation. In the
future, this approach will be tested on real depolymerized mixed waste fractions to further advance the recycling of plastic waste.
Acknowledgement:
This work was funded by the European Union’s Horizon 2020 research and innovation programme as part of the project CIRCULAR FOAM under grant agreement No. 101036854. We also acknowledge support of the Werner Siemens Foundation in the frame of the WSS Research Centre “catalaix”.