Synergistic effect of diatomaceous earth as a catalyst and terephthalic acid as a co-catalyst on chemical recycling of polyethylene terephthalate via hydrolysis

Abstract

Polyethylene terephthalate (PET) is one of the most useful commodity polymers that have been used extensively in packaging and textile industry and hence it has become a concerning waste. In this work, diatomaceous earth (DE), terephthalic acid (TPA) and distilled water were used to hydrolyze PET waste from drinking water bottles into TPA and ethylene glycol (EG). TPA can be used as one of the monomers for the synthesis of PET or other industrial applications such as for the synthesis of alkyd resins. Here DE was used as a solid catalyst which can be separated by a simple filtration after the TPA removal, in contrast to the majority of earlier efforts, which used aqueous acid or base catalysts that are difficult to separate. Additionally, commercial TPA, which is used as co-catalyst, can easily be substituted by produced TPA from the PET hydrolysis. The conversion of PET and the yield of TPA, using 5.0 wt % of DE and 3.0 wt % of commercial TPA with a PET:H2O mass ratio of 1.0:3.0, at 185 °C and 11.2 bar was 100 % and 95.40 % respectively, obtained in 2.5 h. TPA product was isolated from the reaction mixture by basic wash and subsequent neutralization and it was characterized by infrared spectroscopy, nuclear magnetic resonance, differential scanning calorimetry, X-ray diffraction and thermogravimetric analysis. The effects of reaction parameters such as reaction temperature, reaction time, mass ratio of PET:H2O and the catalysts loading on the yield of TPA were studied. The optimized reaction was repeated over seven consecutive reaction cycles using recovered DE to further demonstrate the effectiveness of reused DE and it was found that the outcomes were unaltered. Due to DE's affordability and accessibility, this reaction is a green and convenient way to handle the staggering accumulation of PET waste.