Professor Kotohiro Nomura's research team at Tokyo Metropolitan University has pioneered an efficient technique for the exclusive depolymerization of PET (polyethylene terephthalate), which includes PET bottles and textile waste. This method utilizes alcohols and an inexpensive, readily available, Earth-abundant iron catalyst, presenting a new avenue for the selective chemical conversion of polyester—an essential technology for a circular economy.

Source: PHYS

Addressing the Plastic Waste Crisis

Plastic waste poses a significant environmental challenge that requires immediate attention. Currently, most plastic is repurposed as fuel, resulting in a low percentage of material being recycled effectively.

Polyester, formed through repeated "ester bonds" from the reaction between a carboxylic acid and an alcohol—specifically PET—has become a widely utilized commodity plastic found in bottles, textiles, carpets, curtains, and more. Although PET bottles are primarily recycled through material recycling (collection, sorting, and reuse), the quality of the resulting recycled resin often falls short compared to virgin resin.

Consequently, there is an increasing recognition of the need to develop a "chemical recycling" method that breaks down ester bonds and efficiently reverts them to chemical raw materials. Traditional methods, however, typically require high temperatures and substantial amounts of acid and/or inorganic or organic bases. Thus, a straightforward, cost-effective, and environmentally friendly approach is highly sought after.

Breakthrough in Depolymerization Technique

The research team has successfully created a simple, acid- and base-free method for the quantitative chemical recycling of PET waste bottles and textile waste through depolymerization with alcohol. This process employs an iron catalyst system, yielding the corresponding terephthalic acid diesters (such as dimethyl terephthalate [DMT], diethyl terephthalate [DET], and bis(hydroxyethyl) terephthalate [BHET]) with an impressive selectivity of 99.7% to 99.9%, even under scale-up conditions. The introduction of a tiny amount of amine enhanced catalytic activity without sacrificing selectivity.

Catalytic Depolymerization Process

The catalyst system, which consists of iron(III) chloride (FeCl3)—an inexpensive and widely available compound—and a specific amine, demonstrated outstanding catalytic performance at temperatures between 120°C and 180°C.

This method also allows for the selective depolymerization of PET when mixed with cotton and other plastics. The exclusive chemical recycling of PET from plastic waste represents a promising solution for advancing a circular economy.