Urea as an efficient and reusable catalyst for the glycolysis of poly(ethylene terephthalate) wastes and the role of hydrogen bond in this process

Abstract

Metal salts and solid super acids are the common catalysts used in the glycolysis of poly(ethylene terephthalate)(PET) wastes to obtain monomer bis(hydroxyethyl) terephthalate (BHET) with high conversion of PET and selectivity of BHET, but there are some drawbacks, such as severe reaction conditions, slow reaction rates and difficult recycling of the catalyst. In this study, urea, as a green, highly active, low-priced, extensive and easily produced catalyst, has been used for the degradation of PET to give BHET. The experimental results show that the catalyst has a high catalytic activity, fast reaction rate and is easily separated from the product. Then the effects of experimental parameters on the conversion of PET and the selectivity of BHET were investigated. The conversion of PET wastes is able to reach up to 100% under the conditions of m(PET wastes) : m(ethylene glycol (EG)) : m(urea) = 1 : 4 : 0.1, atmosphere pressure at 160 °C for 2.5 h. The recycling experiments showed that urea worked efficiently, even though it has been used ten times. In addition, the mechanism was proposed through in situ IR, density functional theory (DFT) calculations and experimental results. It shows that the hydrogen bonds (H-bonds) formed between EG and urea play a key role. Following on from this, five tetraalkylammonium-based amino acid-functionalized ionic liquids were synthesized and were used in the catalytic glycolysis of PET. The experimental results and DFT calculations further verify the important role of H-bonds in the glycolysis process of PET catalyzed by this type of carbonyl-containing catalysts.