The contribution of high-resolution GC separations in plastic recycling research

Abstract

One convenient strategy to reduce environmental impact and pollution involves the reuse and revalorization of waste produced by modern society. Nowadays, global plastic production has reached 367 million tons per year and because of their durable nature, their recycling is fundamental for the achievement of the circular economy objective. In closing the loop of plastics, advanced recycling, i.e., the breakdown of plastics into their building blocks and their transformation into valuable secondary raw materials, is a promising management option for post-consumer plastic waste. The most valuable product from advanced recycling is a fluid hydrocarbon stream (or pyrolysis oil) which represents the feedstock for further refinement and processing into new plastics. In this context, gas chromatography is currently playing an important role since it is being used to study the pyrolysis oils, as well as any organic contaminants, and it can be considered a high-resolution separation technique, able to provide the molecular composition of such complex samples. This information significantly helps to tailor the pyrolysis process to produce high-quality feedstocks. In addition, the detection of contaminants (i.e., heteroatom-containing compounds) is crucial to avoid catalytic deterioration and to implement and design further purification processes. The current review highlights the importance of molecular characterization of waste stream products, and particularly the pyrolysis oils obtained from waste plastics. An overview of relevant applications published recently will be provided, and the potential of comprehensive two-dimensional gas chromatography, which represents the natural evolution of gas chromatography into a higher-resolution technique, will be underlined.

Keywords: Comprehensive two-dimensional gas chromatography; Gas chromatography; Hydrocarbons; Mass spectrometry; Molecular characterization; Plastic waste composition; Pyrolysis oil.

Fig. 1

Lifecycle of polymer materials. Depending on the chosen route, polymers will end up in recycling, energy recovery, or landfill, with permission from [1]

Fig. 2

Overview of the plastic recycling techniques from [7]

Fig. 3

Temporal trends of research articles related to plastic pyrolysis research. The bar graph relates to the number of published papers and the continuous line is the moving average. The inset reports the number of publishing authors on the same temporal window. Keywords: plastic pyrolysis oil (and) GC (or) gas chromatography. Source: Scopus

Fig. 4

A Thermal desorption chromatogram at 300 °C; B Pyrogram at 750 °C of chewing gum; C Corresponding MS ion current at 205 m/z; D corresponding MS ion current at 149 m/z. Reproduced with permission from [41, 42]

Fig. 5

GC-VUV chromatograms (average 130–240 nm) of PO blend ranged from C₄–C₃₂. Reproduced with permission from [16]

Fig. 6

2D plots obtained from the analysis of landfill (top), plastic (middle), and pine (bottom) POs. The marks and dotted area highlight some of the constituents and chemical classes. Reproduced with permission from [53]

Fig. 7

A GC×GC-FID chromatogram of MPO rigids PO; B PIONA analysis of the PO using GC×GC-FID. Reproduced with permission from [15]

Fig. 8

A GC–MS chromatogram expansion of the C₄₀ group from the pyrolysis of a PP and PP/PS (reproduced with permission from [51]); B GC×GC-PI-HRTOFMS chromatogram expansion of the C₂₁ group (adapted from [58])

Fig. 9

Occurrence ratios (OR) of 1123 chemicals measured in virgin and recycled products. OR is defined as the fraction of recycled products with occurrence divided by the fraction of virgin products with occurrence. The size of the points indicates the number of individual products with occurrence; the color indicates the number of product categories overall with at least one occurrence. Chemicals with a high occurrence ratio (> 4) are labeled. Points at the very top and bottom of the graph are chemicals with an OR of infinity (no occurrence in virgin products) and 0 (no occurrence in recycled products), respectively; these chemicals occurred in relatively few products and categories. Reproduced with permission from [60]