(505g) Characterization of Plastic-Based Pyrolysis Oils: A Comparison between One-Dimensional Gas Chromatography and Two-Dimensional Gas Chromatography

The U.S. produced 35.67 million tons of plastic waste in 2018, of which 3.09 million tons were recycled—while 26.97 million tons of plastic waste were landfilled, corresponding to 75% of all plastic waste produced in the U.S. in 2018. Polypropylene (PP), high-density polyethylene (HDPE), and polystyrene (PS) play a vital role across various applications, including durable goods, automotive components, containers, and packaging. The continuous use of PP, HDPE, and PS has resulted in a consequential surge in plastic waste, posing challenges in effective waste management and recycling. Among chemical recycling, pyrolysis stands out as a promising technique. It involves the decomposition of polymers by heating to a moderate temperature (400 – 600˚C) under a non-reactive atmosphere in the absence or presence of catalysts. The pyrolysis oil can be further utilized by feeding it into steam cracker. However, pyrolysis oil resulted in intricate reaction networks influenced by sample characteristics, pyrolysis temperature, sample size, and carrier gas flow rate. These factors contribute to the complexity of reaction pathways, which may result in limitations related to kinetic measurements and transport phenomena if these factors are overlooked.

To evaluate the abovementioned parameters, it is necessary to have a comprehensive understanding of the pyrolysis product speciation data. Moreover, the speciation data for the pyrolysis of polymer are fundamental to assessing (micro-) kinetic models, which need to be validated with data obtained under well-defined pyrolysis conditions and a kinetically controlled regime. Traditionally, pyrolysis studies have employed one-dimensional gas chromatography (GC) coupled to mass spectrometry (GC–MS) or flame ionization detector (GC–FID). Nevertheless, peak resolution is compromised when GC is used for complex matrices such as pyrolysis oils. While GC may result in complicated chromatograms with overlapping peaks failing the identification and quantification of pyrolysis products, two-dimensional gas chromatography (GC×GC) offers enhanced resolution, achieved by a two-column combination with different separation mechanisms connected in series through a modulator. GCGC can be coupled simultaneously to FID and MS, such as a time-of-flight mass spectrometer (GC×GC–FID/TOF–MS).

This work examined the product distribution of plastic-based pyrolysis oil by using two analytical techniques: GC–MS and GC×GC–FID/TOF–MS. The pyrolysis of the single polymers (i.e., PP, HDPE, and PS) and their co-pyrolysis were conducted in a micropyrolyzer coupled to GC–MS and GC×GC–FID/TOF–MS at a pyrolysis temperature of 500 ^oC. The total yield for the pyrolysis of for PP and PS was 43.9 ± 0.7 and 86.9 ± 2.3 wt.%, respectively. Meanwhile, the co-pyrolysis of PP-PS yielded 64.0 ± 1.4 wt.%. GC×GC identified up to 6 times more peaks compared to GC, improving the identification of pyrolysis products with higher polarity (i.e., aromatics) using a normal column combination. Moreover, new pyrolysis products were identified using GC×GC. The new products identified corresponded diolefins, cyclic hydrocarbons, and aromatic hydrocarbons. For instance, new products identified using GC×GC in the pyrolysis of PP yielded up to 1.3 wt.%. These chemical classes may be produced by secondary reactions induced by transport effects as well as higher vapor residence times; however, GC overlooks these chemical classes due to inferior resolution compared to GC×GC. It is worth noting that under the low gas residence times investigated in this study, secondary reactions were not prominent. Nonetheless, the high resolution and sensitivity of GC×GC facilitated the detection of these components. Besides, a clear enhancement in the identification of the pyrolysis products occurred when GC×GC was used to analyze the pyrolysis oil products from the co-pyrolysis. For example, GC×GC was able to identify 38 new products from the co-pyrolysis of PP-PS, which corresponded to a yield of 0.5 ± 0.07 wt.%. On the other hand, these products were not identified using GC due to low intensity and co-elution of the GC chromatogram. Considering the possible increased extent of secondary reactions at larger scale reactors, utilization of GC×GC analysis could become more essential. The findings of this work clearly show that a comprehensive knowledge of the product distribution of plastic pyrolysis oils is fundamental to acknowledging unwanted by-products and, therefore, assessing precise (micro-) kinetic models.