Highly efficient removal of aromatic diamines from the polyurethane bio-hydrolysate by MIL-53 series MOFs

Due to the biotoxicity and ecological hazards of 2,4-toluene diamine (TDA) and 4,4′-methylene dianiline (MDA), it is the key process for upcycling polyurethane (PU) waste to remove them from the PU bio-hydrolysate with high selectivity and efficiency. Herein, MIL-53 series metal–organic frameworks (MOFs) were prepared and applied to remove aromatic diamines. Besides, the effects of their different structural units on the adsorption performance were investigated. Among the three metal nodes (Cr3+, Al3+, Fe3+) of MIL-53, MIL-53(Al) demonstrated the highest adsorption capacities upon TDA (4.17 mmol/g) and MDA (2.00 mmol/g) due to its more μ2-OH sites and higher porosity. Compared with MIL-53(Al), DUT-4 and DUT-5 owned longer linkers but exhibited inferior aromatic diamine adsorption performance, which was ascribed to the more appropriate pore size and more μ2-OH sites of MIL-53(Al). Notably, MIL-53(Al) exhibited excellent selectivity towards aromatic diamines, while the removal efficiency reached over 90 %. In addition, spectral analysis before and after adsorption revealed the mechanism, which involved host–guest interactions, H-bonding interactions, and π-π stacking. The results indicate the great potential of applying the optimal MIL-53(Al) as an alternative adsorbent for removing hazardous aromatic diamines from the PU bio-hydrolysate.