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

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.

Scheme 1. Scheme for Oxidative PE Conversion to Dicarboxylic Acids (a), Which can Subsequently be Converted into Gaseous Hydrocarbon Products via Photocatalysis to Give Alkanes (b) or Electrolysis to Produce Alkenes (c)

Figure 1

Product yields of photocatalytic experiments with (a) P25|Pt and (b) ^NCNCN_x|Pt. Conditions: AM1.5G, 100 mW cm^–2, 25 °C, 2 mg mL^–1 photocatalyst, and 2 mL of 10 mg mL^–1 succinic acid in 0.1 M HNO₃ set to pH 4.

Figure 2

(a,b) Product yields from photocatalytic experiments using a flow setup with an irradiated area of 25 cm² with (a) P25|Pt or (b) ^NCNCN_x|Pt deposited on glass sheets. Conditions: AM1.5G, 100 mW cm^–2, backside irradiation, 25 °C, and 50 mL of PE decomposition solution. As the reaction time includes circulation through the reactor and the reservoir, the actual irradiation duration (residence time) is only 0.6 times the reaction time. For each equivalent of hydrocarbon, two equivalents of CO₂ are expected to be formed (Figure S5), but the amounts were below the limit of quantification due to the large volume of the reservoir. (c) Photographic image of the photocatalytic flow setup. The PE decomposition solution (not shown in picture) is continuously pumped from a reservoir using a peristaltic pump into the photoreactor (25 cm² irradiated area) before returning to the reservoir. Evolved gaseous products are sampled and analyzed by GC.

Figure 3

Electrolysis with (a) succinic acid solution, 3.3 mg mL^–1, in 0.1 M aqueous HNO₃ set to pH 10 by addition of NaOH or in 2:1 methanol/0.1 M aq. HNO₃ (set to pH 10 with NaOH) solutions; (b) PE decomposition solution set to pH 10 or 2:1 diluted with methanol. Working electrode: carbon paper (2 cm² electrode area), counter electrode: Pt foil (2 cm² electrode area), applied voltage was 5 V until approximately 130 C (= 1 Faradaic equivalent) has been passed through the cell. For each equivalent of hydrocarbon, two equivalents of CO₂ are expected to be formed from the decarboxylation reaction (Figure S11).