Biocatalysis as a green route for recycling the recalcitrant plastic polyethylene terephthalate

TLDR: Biocatalysis can enable a closed‐loop recycling of post‐consumer PET waste and help in the development of new materials for recycling PET waste recycling.

Abstract: The global production of fossil‐based plastics has grown more than 20‐fold since 1964 to 322 million tons in 2015, and a slowdown of this rate is not expected (Ellen MacArthur Foundation and World Economic Forum, 2014; PlasticsEurope, 2016). Many materials derived from synthetic polymers have already replaced their natural counterparts in all areas of human life. The majority of plastics are short‐lived products which are disposed within 1 year after manufacture. However, only 14% of plastic packaging materials used worldwide is currently collected for recycling while another 14% is incinerated for energy recovery (Ellen MacArthur Foundation and World Economic Forum, 2014). The remaining 72% of plastic packaging is not recovered with 40% land filled and 32% estimated to completely escape the collection system. This part of plastic waste ends up in diverse natural habitats, especially in oceans where it can cause serious environmental damages (Andrady, 2015; Jambeck et al., 2015). Therefore, innovative technologies to improve the recycling of plastics and to reduce the consumption of non‐renewable fossil feed stocks are required. Polyethylene terephthalate (PET) is the most widely used synthetic polyester. It is a thermoplastic of high‐molecular‐weight composed of terephthalic acid (TPA) and ethylene glycol (EG). PET can exist as both an amorphous and a semi‐crystalline polymer (Webb et al., 2013). Owing to its excellent physical and chemical properties, PET finds numerous applications as textile fibres, packaging materials and beverage bottles. PET is generally referred to as ′polyester′ in the textile industry which consumes the majority of the PET produced globally. In 2014, 49.2 million tons of PET fibres was produced worldwide (Fiber Economics Bureau, 2015). In 2015, the global production of PET resins was 27.8 million tons which was dominantly used for the manufacture of packaging materials and beverage bottles (Plastic Insight, 2016). Almost half of the postconsumer PET bottles worldwide are collected for mechanical recycling to produce polyester fibres (Ellen MacArthur Foundation and World Economic Forum, 2014). Polyethylene terephthalate made from renewable biomass (bio‐PET) is becoming of industrial interest lately. EG derived from sugarcane ethanol (Tsiropoulos et al., 2015) and TPA derived from sugar beet paraxylene (Collias et al., 2014; Smith, 2015) can be utilized to replace their fossil‐based counterparts to produce bottle‐grade PET. Although fossil feedstocks can be saved by the commercialization of bio‐PET bottles, the challenges for their recycling remain as their recalcitrant properties are the same as those of petroleum‐derived PET bottles (Chen et al., 2016).