Selective Separation of 4,4'-Methylenedianiline, Isophoronediamine and 2,4-Toluenediamine from Enzymatic Hydrolysis Solutions of Polyurethane

The recycling of plastics such as polyurethane (PU) is a current challenge. The continuously increasing production volume and additionally growing end-of-life streams increase the urgency for solutions. Conventional recycling methods such as mechanical and chemical recycling are only economically and/or ecologically suitable to a limited extent. The three-step approach consisting of enzymatic PU decomposition, separation of amines (4,4'-methylenedianiline (MDA), isophoronediamine (IPDA), 2,4-toluenediamine (TDA)) and fermentation of the residual stream is a promising recycling concept. In this study, extraction methods for the separation of MDA and IPDA from an aqueous solution at neutral pH are developed. In addition, the influence of relevant PU hydrolysate components on the amine extraction is investigated. The results show that MDA can be efficiently separated using solvent extraction of 1-octanol. For IPDA separation, a reactive extraction with oleic acid as reactant is developed. The application of these two extraction methods to TDA shows extraction efficiencies of 52% to 86%. The other PU hydrolysate components adipic acid and selected salts have only a minor influence on the extraction efficiency. The diols ethylene glycol and 1,4-butanediol influence the equilibrium pH of IPDA extraction, raising it to higher values. For MDA, no influence of other PU hydrolysate components on the extraction efficiency can be observed. Since amines can have an inhibitory effect on microorganisms, toxicity experiments were carried out to determine the tolerable residual concentration of amines in the raffinate to avoid fermentation inhibition. Growth experiments with Pseudomonas putida KT240 show that MDA has an inhibitory effect at concentrations near the solubility limit, whereas IPDA does not affect growth.