Conversion of Polyethylene Waste into Gaseous Hydrocarbons via Integrated Tandem Chemical–Photo/Electrocatalytic Processes

Christian M Pichler,Erwin Reisner,Motiar Rahaman,Subhajit Bhattacharjee,Taylor Uekert

doi:10.1021/acscatal.1c02133

Christian M Pichler, Erwin Reisner + Show 3 more

Open Access

https://doi.org/10.1021/acscatal.1c02133

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Journal: ACS Catalysis	Publication Date: Jul 9, 2021
Citations: 63	License type: CC BY 4.0

Affiliation: University of Cambridge

Abstract

The chemical inertness of polyethylene makes chemical recycling challenging and motivates the development of new catalytic innovations to mitigate polymer waste. Current chemical recycling methods yield a complex mixture of liquid products, which is challenging to utilize in subsequent processes. Here, we present an oxidative depolymerization step utilizing diluted nitric acid to convert polyethylene into organic acids (40% organic acid yield), which can be coupled to a photo- or electrocatalytic decarboxylation reaction to produce hydrocarbons (individual hydrocarbon yields of 3 and 20%, respectively) with H2 and CO2 as gaseous byproducts. The integrated tandem process allows for the direct conversion of polyethylene into gaseous hydrocarbon products with an overall hydrocarbon yield of 1.0% for the oxidative/photocatalytic route and 7.6% for the oxidative/electrolytic route. The product selectivity is tunable with photocatalysis using TiO2 or carbon nitride, yielding alkanes (ethane and propane), whereas electrocatalysis on carbon electrodes produces alkenes (ethylene and propylene). This two-step recycling process of plastics can use sunlight or renewable electricity to convert polyethylene into valuable, easily separable, gaseous platform chemicals.

Highlights

Reducing the amount of waste plastics and mitigating its disposal in landfills and escape into the environment are important contemporary challenges.[1]
We show that distinct differences between the photocatalytic and electrocatalytic mechanisms lead to different product selectivities, with hydrogen being co-generated as fuel in both systems
The conversion of the carboxylic acid intermediates into gaseous products removes the requirement for complicated liquid-phase separation, and our integrated two-stage process serves as a first step toward circular chemical recycling of PE, which has the potential to significantly reduce the quantity of PE waste disposed in landfills or released into the environment (Scheme 1)

Summary

■ INTRODUCTION

Reducing the amount of waste plastics and mitigating its disposal in landfills and escape into the environment are important contemporary challenges.[1]. We report a two-step process that combines an established polymer oxidative process with photocatalytic and electrocatalytic routes to convert PE via carboxylic acid intermediates into gaseous hydrocarbon products that can serve as precursors for the synthesis of new plastics or other valuable organics (e.g., ethylene oxide and vinylchloride). The conversion of the carboxylic acid intermediates into gaseous products removes the requirement for complicated liquid-phase separation, and our integrated two-stage process serves as a first step toward circular chemical recycling of PE, which has the potential to significantly reduce the quantity of PE waste disposed in landfills or released into the environment (Scheme 1)

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES