Acid-modified, Ti3C2-based MXene as catalysts for up-cycling polyethylene terephthalate

Plastics are indispensable materials for packaging and many products from our daily life and their recycling is essential to ensure a circular economy. In this study, −SO3H modified-MXene, Ti3C2, was used as a recoverable solid acid catalyst for up-cycling of polyethylene terephthalate (PET) to terephthalic acid (TPA) and ethylene glycol (EG) by hydrolysis. For the addition of −SO3H groups to the Ti3C2Tx surface (where Tx represents the surface moieties such as -OH or -O), sulfonation with an aryl diazonium salt obtained from sulfanilic acid was employed. X-ray photoelectron and Fourier transform infrared spectroscopies analyses provided a direct indication that sulfonation of the Ti3C2Tx was successfully performed, while X-ray diffraction and Transmission electron microscopy analyses evidence the presence of −SO3H groups in between the layers of Ti3C2Tx due to the increases of the interlayer spacing through the intercalation of functional groups. The higher the concentration of acid groups, the higher the interlayer spacing. The depolymerisation of PET in water occurred with a very good isolated yield in TPA (99%) for the MXene with the highest amount of sulfonic acid groups. We can conclude that the acidity is mandatory to perform the hydrolysis reaction, in agreement with the acidity measurements, which shows that the MXene modified with the highest amount of derived sulfonic acids contains the highest amount of acidity. Nevertheless, the accessibility to the acidic sites is a key factor that promotes the 2D acid-modified MXene materials as important catalysts for PET up-cycling to TPA.