Insights into 1-ethyl 3-methyl imidazolium dicyanamide-catalysed glycolysis of polyethylene terephthalate

Abstract

The non-biodegradable nature of polyethylene terephthalate (PET) raises concerns about the accumulation of post-consumer PET products in the environment. Recycling is the most viable solution to address this issue. Among PET recycling methods, glycolysis has gained commercial interest due to favourable reaction conditions. Ionic liquids (ILs) have emerged as promising green catalysts for PET glycolysis. This study highlights the use of 1-ethyl 3-methyl imidazolium dicyanamide ([EMIm][DCA]) ionic liquid as an efficient catalyst for PET degradation. Probable reaction mechanisms and side reactions encountered were also discussed. Under optimised reaction conditions, a bis-hydroxy ethylene terephthalate (BHET) yield of 93% and 100% PET conversion were achieved. A BHET yield of over 57% was obtained in just 0.5 hours. Neutral hydrolysis of the ester bond was identified as a significant side reaction at high concentrations of [EMIm][DCA]. This process results in the formation of monohydroxy ethylene terephthalate (MHET), which is also a valuable monomer for various synthesis applications. The possible side reactions and reaction products were identified and quantified by HPLC, NMR spectroscopy and chemical titrations. IL catalysed glycolysis reaction was found to follow the pseudo first order kinetics with an experimental activation energy of 168 kJ mol⁻¹. The mechanism of catalysis was explored through DFT studies, revealing the key role of hydrogen bonding interactions between IL and ethylene glycol (EG) in catalysing glycolysis reactions. DFT and spectroscopic methods confirmed the hydrogen bonding between EG and [EMIm][DCA] with a computed interaction energy of –118 kJ mol⁻¹. Based on DFT studies, a novel two-step mechanism was proposed for the glycolysis. The computed activation barrier for the catalysed glycolysis is 166 kJ mol⁻¹ against 204 kJ mol⁻¹ for the uncatalyzed one.