Full length articleEfficient chemical recycling of waste polyethylene terephthalate
Graphical abstract
Introduction
In today's modern age, plastics fulfill indispensable roles in our everyday lives thanks to their unparalleled performance, ease of processing, and affordability.(Andrady and Neal, 2009; Khan et al., 2020) Present estimates suggest that plastic production has exceeded 395 million tons/year.(Geyer et al., 2017; Kumar et al., 2022) After use, the majority of discarded plastics accumulate in the natural environment (e.g., landfills).(Geyer et al., 2017) Among these plastics, polyethylene terephthalate (PET) is the most abundant polyester, with an annual production of approximately 70 million tons worldwide for use in packaging and redundant textiles.(Tournier et al., 2020) This rapid growth in plastic usage, combined with pressure on waste management infrastructure worldwide, has created a daunting environmental crisis.(Boucher and Billard, 2019) Consequently, there is an urgent need for an effective solution to handle plastic waste.(Galloway et al., 2020; Sheldon and Norton, 2020)
PET can be recycled via mechanical and chemical recycling methods.(Khoonkari et al., 2015; Schyns and Shaver, 2021) Mechanical recycling of recycled PET (r-PET) is more economical than chemical recycling.(Schyns and Shaver, 2021) However, some disadvantages are associated with the mechanical recycling of r-PET, such as loss of the polymer's molecular weight (Mw) and reduction in viscosity due to polymer chain scissions during the melt-processing, which hence weakens the overall mechanical properties of the r-PET.(Dimonie et al., 2012; López et al., 2014; Vollmer et al., 2020) Despite the use of chain extension additives, solid-state polymerization approaches and addition of virgin PET (petrochemically-derived) to enhance the mechanical properties of these materials, 79% of the r-PET produced still ends up in downcycled products (e.g., fiber) that will be destined to landfills at the end of their service life (linear economy).(Ügdüler et al., 2020; Wu, 2021)
On the other hand, chemical recycling of r-PET (depolymerization into monomers) has many advantages such as offering a true circular economy approach (e.g., bottle-to-bottle recycling for a virtually unlimited number of cycles), providing similar properties to those of virgin PET, lack of impurities, and hence yielding materials that are suitable for food contact packaging applications.(Bartolome et al., 2012; Khoonkari et al., 2015; Paszun and Spychaj, 1997; Payne and Jones, 2021) However, the current approaches used for the chemical recycling of r-PET yield feedstock monomers that are more expensive than those of their petrochemical-derived counterparts, thus making chemical recycling a cost-prohibitive process. Multiple factors contribute to the costs associated with chemical recycling, including but not limited to, energy-intensive depolymerization performed at higher temperatures and for longer times.(Achilias and Karayannidis, 2004; Barnard et al., 2021; Bartolome et al., 2012; Sinha et al., 2010), The low cost of petrochemical-derived monomers for virgin PET has impeded the chemical recycling of r-PET. Consequently, r-PET is mechanically downcycled to fibers, which in turn end up in landfills or incinerators.(Geyer et al., 2016; Sandin and Peters, 2018) Consequently, r-PET is mechanically downcycled to fibers, which in turn end up in landfills or incinerators.(Geyer et al., 2016; Sandin and Peters, 2018) As for a sustainable circular plastic economy, it is imperative to develop economically sound methods for the chemical recycling of r-PET by addressing the prevailing challenges.(Kosloski-Oh et al., 2021) In this direction, some landmark studies have been reported. For example, ionic liquids-based catalytic depolymerization has been intensively studied for rapid depolymerization of PET.(Wang et al., 2009) In a remarkable study, amidine organocatalyst for the rapid depolymerization of PET, where the catalyst can be distilled and recovered.(Fukushima et al., 2013) Nevertheless, the existing chemical recycling is yet to meet to cost-competitiveness because of energy intensiveness nature and the use of large doses of catalyst which must be removed at the end of depolymerization.(Payne and Jones, 2021)
Enzymatic depolymerization of PET back to its parent monomers is getting very popular mainly due to the greener and scalable nature of this method.(Chen et al., 2020) Enzymatic degradation is a suitable method for all polyesters because of their ester linkages that undergo degradation when exposed to ester hydrolase.(Ghosal and Nayak, 2022) However, enzymatic depolymerization is hindered by sensitivity to pH and temperature and the low rate of depolymerization for crystalline polymers.(Taniguchi et al., 2019) Recently, Alper et al.,(Lu et al., 2022) reported the use of machine learning to create PET hydrolase that is more efficient and robust. The method was successfully implemented for the depolymerization of amorphous PET (thermoform). The crystalline PET was difficult to depolymerize without pre-treament. Nevertheless, the state-of-the-art enzymatic methods take several days to depolymerize bottle PET at 70C, which is time and energy less efficient. More research in this direction is ongoing.
Herein we report a method enabling the rapid depolymerization of r-PET under mild conditions without using any corrosive organic solvent or special catalysts. One novelty of this approach is that the chemical depolymerization of r-PET can be accelerated by suppressing the difficult-to-depolymerize crystalline domains in r-PET via thermal-quenching treatment. The second novelty of this strategy is the use of catalytic nano-sites where the diols in the presence of catalyst triggers the depolymerization of r-PET from the bulk (Fig. 1A). These facile approaches enabled complete depolymerization of r-PET into monomers much faster compared to that achieved with a control sample by 50-fold and 8-fold energy saving at 170 °C and 200 °C, respectively.
Section snippets
Materials
PET coke bottles (SP code #1) were purchased from local Meijer store at Michigan, USA. Zinc acetate (MilliporeSigma), Zinc 2-ethyl hexanoate (AlfaAesar), Bis (2-hydroxyethyl) terephthalate (BHET, MilliporeSigma), 1,4-Cyclohexanedimethanol (CHDM, MilliporeSigma), 4,8-Bis(hydroxymethyl)tricyclo[5.2.1.02,6]decane (BHTD, MilliporeSigma), Methanol (MeOH, 99.8%, MilliporeSigma), Dimethyl terephthalate (DMT, MilliporeSigma) were used as received.
General procedure for the extrusion
The extrusion was performed using DSM Xplore 15cc Micro
Results and discussion
PET is a semi-crystalline polymer and thus bears amorphous and crystalline domains.24 During depolymerization, crystalline regions of PET offer resistance to mass transfer of any reagents such as methanol, water, ethylene glycol, or ammonia. Consequently, the depolymerization of PET is an energy-intensive and lengthy process. The main goal of this study was to develop transformative methodologies for the chemical recycling of PET by suppressing the crystalline domains and creating
Conclusions
In summary, we have demonstrated a rapid and efficient depolymerization approach for r-PET. The depolymerization time of r-PET samples which had only been subjected to melt-quench treatment decreased by 30–50%. Interestingly, when the pretreatment was performed in the presence of catalyst and diol (aka ‘nano-site’), the depolymerization time was decreased by 5 folds owing to the decrease in Tm, broadening of molecular weights caused by chain scission, and the occurrence of depolymerization
CRediT authorship contribution statement
Ajmir Khan: Performed & designed experiments, collected the data, performed characterization & analysis, and writing-original draft preparation. Muhammad Naveed: Performed the extrusions. Zahra Aayanifard: Energy calculations, contributed data for some of the experiments. Muhammad Rabnwaz: Conceived the idea, supervision and writing-reviewing and editing.
Data and materials availability
Data needed to evaluate the conclusions in the paper is provided in Supplementary Information (SI). Experimental procedures of the depolymerization reactions, extrusion protocols, and energy calculations are included in the SI.
Authors are thankful to the MSU Mass Spectrometry and Metabolomics Core for GC–MS analysis.
Declaration of Competing Interest
The author has filed a US patent application no. 63/171,710. The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Muhammad Rabnawaz, Ajmir Khan, Zahra Aayanifard has patent pending to Michigan State University.
Acknowledgements
None.
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