Highlights
- Biodegradability helps alleviate the environmental impact of plastics.
- Recycling biodegradable plastics helps achieve a circular economy.
- Most biodegradable plastics cannot decompose in the natural environment.
- Efficient recycling methods limit the degradation of plastics in the environment.
- Catalytic chemical recycling permits selective/efficient recycling of bioplastics.
Abstract
The widespread utilization of plastics results in millions of tons of waste plastics being generated every year. During the last decade, converting waste plastics to valuable resources has been a primary objective for researchers in academia and industry.
Nonetheless, most of the plastic recycling methods still rely on mechanical processes that are limited to relatively pure polymers; these mechanical processes generate end-products with lower thermo-mechanical properties.
Therefore, tertiary recycling or chemical recycling methods are currently gaining more interest, mainly with conventional petroleum-based polymers.
Recently some examples of catalytic chemical recycling have been described on biodegradable plastics, especially thermoplastics. Indeed, the recycling of biodegradable plastics helps to close the loop towards a lower environmental impact in the life cycle assessment of the material.
The purpose of this review is to present the latest catalytic chemical recycling methods that were developed as solutions to the end of life management of biodegradable polyesters, with a focus on polylactic acid (PLA) and polyhydroxybutyrate (PHB).
Conclusion
This review highlights the most significant progress in catalytic recycling of biodegradable polyesters. Four routes for polyester degradation are presented: alcoholysis, hydrolysis, enzymatic degradation and reductive depolymerization.
These different degradation routes allow the transformation of waste plastic into high value-added products.
Alcoholysis is a known approach for the depolymerization of petroleum derived oxygenated polymers such as PET and polycarbonates. Not surprisingly, it has also been recently extended to biodegradable polyesters.
The types of catalyst described for the transformation of PLA to methyl lactate include metal catalysts, organocatalysts and ionic liquids.
On the contrary, the methanolysis of PHB is less well explored, and so far, most reports have been focused on obtaining value-added molecules from the polymer rather than recovering the monomers for further recycling.
There is a growing interest in acid, alkaline and neutral hydrolysis of biodegradable polyesters. This interest is due to the increased use of polyesters as bioresorbable materials in therapeutic applications and in our everyday plastic based-consumables.
The mechanism of their degradation depends on the environmental conditions such as temperature, pH and polymer crystallinity. The products resulting from their biodegradation could have an important impact on the polymer biocompatibility.
Research on reductive depolymerization of bioplastics has emanated recently in the scientific community. Iridium and ruthenium based organometallic complexes and an organocatalyst (B(C6F5)3) were used in the presence of hydrogen or a hydrosilane as reducing reagents.
Some of the challenges in the reductive depolymerization include the development of highly active recyclable catalysts with excellent selectivity, low cost and operability under mild reaction conditions.
The use of non-green solvent systems e.g. chlorobenzene is also a considerable problem that should be addressed. In addition to the cost efficiency and recyclability of the catalyst, the selectivity towards high value-added products and the easiness of product isolation (hydrolysis, post-treatment) are major drawbacks of catalytic reductive depolymerization.
Nevertheless, the possibility of using hydrogen as a renewable reductive reagent could help to reduce the cost of separation and purification. Research in this field is still essential in order to tackle the challenges related to the sustainability of the chemical industry.
To finish, the use of enzymes as biocatalysts could provide viable alternatives to metal complexes for plastic waste depolymerization.
The lipase enzyme can catalyze both polymer production and depolymerization. Researchers have already shown reversible polymerization-depolymerization of PCL in presence of a biocatalyst. The reversible cycle was controlled by the presence or absence of an organic solvent, toluene.
Although a considerable amount of work has been performed on enzymatic degradation of polymers, they barely provide an insight into the enzyme structure or recognize the subunit that actually accomplish the catalytic degradation.
In order to reach completely sustainable cycles, plastic polymerization, depolymerization and then re-polymerization should be performed with minimal changes in their final properties.
Accordingly, a better understanding of enzymatic reactions will enable their better use as biocatalysts for the recycling of biodegradable polyesters.
Refs and Read more:
Catalytic chemical recycling of biodegradable polyesters
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