A kinetic study of the depolymerisation of poly(ethylene terephthalate) by phase transfer catalysed alkaline hydrolysis

Abstract

BACKGROUND: Chemical or tertiary recycling of waste polymers including PET, poly(ethylene terephthalate), leads to the formation of raw starting monomers by different depolymerisation routes. This work was focused on the identification of the catalytic behaviour, if any, of a series of quaternary phosphonium and ammonium salts as phase transfer catalysts for the alkaline hydrolysis of PET, and on the determination of the kinetics of the phase transfer catalysed process.

Kinetics of catalytic glycolysis of PET wastes with sodium carbonate

“…A transition from a heterogeneous reaction to a homogeneous reaction during depolymerisation was also observed for the non-catalytic glycolysis of PET [16], the methanolysis of PET with supercritical methanol [7], and the non-catalytic glycolysis of polycarbonate [48]. This finding was in stark contrast with the depolymerisation mechanism observed for the alkaline hydrolysis of PET since, apart from being an irreversible reaction, it always occurred in the heterogeneous phase [9].…”

“…Great attention is currently paid to chemical recycling, which basically involves the recovery of monomers leading to the yield of interesting value-added chemicals or intermediates from PET waste [3][4][5][6]. PET is a polyester with functional ester groups that can be cleaved by reagents, such as water, acids or bases (hydrolysis) [7][8][9], alcohols (alcoholysis) [10,11], amines (aminolysis) [12,13], ammonia (ammonolysis) [14] and glycols (glycolysis) [15,16]. Particularly, the glycolysis reaction is the molecular degradation of PET polymer by glycols, typically ethylene glycol (EG), where ester linkages are broken and replaced by hydroxyls terminals to give bis(2-hydroxyethyl terephthalate) (BHET), according to the following reaction scheme,…”

Development of an efficient route for combined recycling of PET and cotton from mixed fabrics

“…Alkaline hydrolysis in sodium hydroxide (NaOH) solutions with the addition of a phase transfer catalyst has been shown to be very effective at depolymerizing PET at moderate temperatures (70-95°C) and alkalinity in the range of 5-15% NaOH (Das et al 2007;Kosmidis et al 2001;López-Fonseca et al 2009;Polk et al 1999). The hydrolysis of PET is shown in Fig.…”

“…The effect of the phase transfer catalyst is, thus, to carry the hydroxide ions to the surface of the PET to facilitate the reaction (Naik and Doraiswamy 1998). Therefore, the phase transfer catalyst should be cationic and have sufficient organic character to be lipophilic, but remain small enough to avoid steric hindrance (López-Fonseca et al 2009;Polk et al 1999). The most common types of phase transfer catalysts are quaternary ammonium or phosphonium salts with lipophilic side chains (Naik and Doraiswamy 1998).…”

“…The most common types of phase transfer catalysts are quaternary ammonium or phosphonium salts with lipophilic side chains (Naik and Doraiswamy 1998). López-Fonseca et al tested the difference between these two types and found only minor differences between phase transfer catalysts based on ammonium or phosphonium ions (López-Fonseca et al 2009). It is, thus, the side chains that are the most important for the effect of the catalyst.…”