Co-Al-CO3 layered double hydroxide: an efficient and regenerable catalyst for glycolysis of polyethylene terephthalate

Abstract

Polyethylene terephthalate (PET) is one among the common polymers we use in our day-to-day lives. Despite its wide range of applications, recycling of PET waste is a serious concern due to its non-biodegradability. This paper deliberates the use of Co-Al-CO₃ layered double hydroxide (LDH) as a catalyst for the glycolysis of PET and the mechanistic aspects of catalysis. Co-Al-CO₃ LDH showed superior properties compared to similar LDH materials. 100% PET conversion and 96% yield for bis hydroxy ethylene terephthalate (BHET) were achieved within a reaction time of 2 hours, at a reaction temperature of 180 °C and with a catalyst concentration of 1%. Glycolysis conditions such as reaction temperature, time, and ethylene glycol (EG)/PET ratio were optimized. The effect of M⁺²/M⁺³ on BHET yield was studied, and an increase in BHET yield was observed up to a ratio of 3 : 1. The replacement of Co²⁺, either completely or partially with another M²⁺, resulted in a significant decrease in BHET yield. The catalysis mechanism of Co-Al-CO₃ LDH was explained by correlating the decarbonation temperature of the carbonate anion with catalytic performance. Magnetically separable CoAl31@Fe₃O₄ was prepared with a BHET yield of 99%. Regeneration was demonstrated up to 4 cycles and a BHET yield of 86% was achieved in the fourth cycle.