Plastics, a pivotal product of the last century, have dramatically transformed industries ranging from healthcare and food safety to textiles, electronics, and construction, making consumer goods more accessible worldwide. Plastic production has soared from its early days, and is projected to continue growing significantly. This increase is primarily due to polyolefins like polyethylene (PE) and polypropylene (PP), which make up about half of all plastic production. However, this manufacturing boom has led to a massive surge in plastic waste, with poor waste management practices causing significant environmental issues. A small portion of this waste is recycled, mainly through mechanical methods that degrade quality.
Chemical recycling to monomers is effective for condensation polymers but less so for polyolefins due to their strong carbon-carbon bonds, often resulting in low-value products. Upcycling is a possible alternative for managing polyolefin waste. The aim is to selectively cleave specific C-C bonds in polyolefins, converting them into building blocks that can be used as feedstocks for chemical manufacturing.
Catalytic conversion of waste polyolefins to value-added alkylaromatics could contribute to carbon recycling. Tandem catalytic conversion by platinum supported on g-alumina converts various polyethylene (PE) grades into valuable long chain alkylaromatics and alkylnaphthalenes (average ~C30, dispersity Ð = 1.1) with high yields, in the absence of added solvent or molecular hydrogen. Coupling exothermic hydrogenolysis with endothermic aromatization renders the overall transformation thermodynamically accessible despite the moderate reaction temperature of 280 °C.
Compared to tandem hydrogenolysis/aromatization of PE catalyzed by Pt/g-Al2O3 at 280°C, both a five-fold enhancement in the rate of C-C bond scission and a doubling of the molar yield of alkylaromatics were realized using a more acidic Pt/F-Al2O3 catalyst instead. Bifunctional (metal/acid) catalysts also generate alkylaromatic products with lower average carbon numbers (ca. C20), similar to conventional anionic surfactants. Since physical mixtures of weakly acidic Pt/g-Al2O3 or non-acidic Pt/SiO2 with strongly Brønsted acidic Cl-Al2O3 or F-Al2O3 are also effective, the tandem reaction does not require nano-scale intimacy between metal and acid active sites. Kinetic studies using triacontane (norm-C30H62) as a model for PE show that the Pt-catalyzed dehydrogenation/hydrogenation reactions are quasi-equilibrated, while the acid-catalyzed C-C bond scission and skeletal transformations (isomerization and cyclization) determine the overall rates of depolymerization and aromatic formation.
Different from hydrocracking, aromatic formation must be conducted without high pressure H2. Depolymerization can be performed without adding any external H2, depending instead on redistribution of PE-derived H2 to value-added alkylbenzenes. However, moderate external H2 pressures (1-8 bar) significantly enhance the depolymerization rate and alkylbenzene yield (maximum yield at 4 bar H2). The rate of C-C bond scission is pseudo-zeroth-order. It increases with external P(H2) at lower pressures (< 10 bar), then decreases at higher pressures. Modifying the traditional hydrocracking mechanism to include competitive adsorption of aromatics explains the P(H2) dependence of the rate. Dynamic control of the reactor atmospheres can further increase the alkylbenzene yield, especially in the desired molecular weight range for anionic surfactants.
Depolymerization and upcycling are promising approaches to managing plastic waste. However, quantitative measurements of reaction rates and analyses of complex hydrocarbon product mixtures arising from the depolymerization of polyolefins constitute significant challenges in this emerging field. The methods for recovery and analysis of all products arising from batch depolymerization of polyethylene are detailed. The quantitative analyses of reaction rates and selectivity for alkanes and aromatic products are described. This protocol can be extended to depolymerization of other plastics such as polypropylene, and to characterization of other product mixtures (including long-chain olefins).
