Upcycling of waste poly(ethylene terephthalate) into value-added materials: a case of iron terephthalate metal-organic frameworks

Upcycling recyclable waste materials into value-added products provides an opportunity to extend the life cycle of valuable resources. This act not only minimizes the exploitation of natural resource reserves but also reduces the volume of solid waste. In this study, waste polyethylene terephthalate (PET) was depolymerized via alkali hydrolysis to reclaim terephthalic acid (H 2 bdc). The reclaimed H 2 bdc was used as a linker to synthesize various iron terephthalate (Febdc) metal-organic frameworks (MOFs). For comparison, MOFs were also synthesized with commercially obtained linker under identical conditions. The effect of three different sources of iron, i.e., anhydrous FeCl 3 , FeCl 3 .6H 2 O and Fe(NO 3 ) 3 .9H 2 O were examined while keeping other synthesis parameters constant. The obtained MOFs were characterized to assess their crystallinity and phase, textural properties, morphology, functional groups, and thermal degradation behavior by powder X-ray diffraction (XRD), N 2 sorption isotherm measurements, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA), respectively. The source of iron was found to influence the formation of a particular phase irrespective of the source of the linker. Specifically, the use of anhydrous FeCl 3 resulted in the formation of MOF-235(Fe) phase while FeCl 3 .6H 2 O yielded MIL-101(Fe) phase and Fe(NO 3 ) 3 .9H 2 O produced nanocrystalline material. However, using HCl as a reaction modulator along with Fe(NO 3 ) 3 .9H 2 O resulted in the formation of crystalline material belonging to MIL-68(Fe) phase. Finally, the synthesized MOFs were examined for CO 2 capture application via gravimetric method. The MOFs obtained from reclaimed linkers showed similarity in their material properties or CO 2 uptake performance compared to their commercial counterpart.