(80a) Detailed Characterization of Plastic Pyrolysis Oils and Their Contaminants By FT-ICR MS and GCxGC

The recycling of plastic waste is a necessity for realizing a circular economy and achieving climate neutrality. Worldwide less than 10% of plastics are currently being recycled using mechanical recycling while chemical recycling is responsible for less than 0.1% of recycled plastics [1]. While mechanical recycling is energy- and resource-efficient it faces challenges such as thermal-mechanical degradation, making the recycled polymer of inferior quality [2]. Moreover, mechanical recycling requires a high-purity plastic waste feed making the processing of heterogeneous waste streams very challenging [3]. Polyolefins (PP, PE) account for 2/3 of the plastic waste volume and cannot be processed via mechanical recycling due to their heterogeneity [4]. Consequently, recycled products are often of lower quality than their virgin counterparts and cannot be used in their original application due to strict regulations in terms of, for instance, food applications [5-7]. Therefore, chemical recycling of these polyolefins is the most promising route to close the loop for plastic waste.

One of the most promising pathways to recycle is the thermal pyrolysis of the plastic waste to produce plastic pyrolysis oils. These pyrolysis oils are a valuable carbon source as they can be converted via various conventional methods to light olefins (and hence recycled to virgin-grade plastics) and base chemicals. These plastic pyrolysis oils contain a wide variety of (un)saturated hydrocarbons and contaminants [8]. However, these contaminants pose several challenges for the processing of pyrolysis oils in the current chemical industry. Kusenberg et al. showed that these oils are unsuitable for direct use in steam cracking with membrane filtration, dehalogenation or hydrotreatment providing a potential solution for this [9]. Furthermore, the presence of these contaminants can poison catalysts in the downstream processing of various (petro-)chemical processes. In addition to the presence of contaminants, plastic pyrolysis oils typically contain a higher fraction of unsaturated hydrocarbons such as olefins, diolefins, and aromatics which make it not straightforward to use them in conventional chemical processes. To facilitate the further processing of this valuable carbon source, it is essential to acquire a proper understanding of their molecular composition. Therefore, in this work three different types of analyses (GCxGC, ICP-OES, FT-ICR MS) were performed on four different pyrolysis oil samples being a virgin LDPE oil, and three post-consumer waste pyrolysis oils of PE, PP and mixed polyolefins (MPO). The combination of the three analyses allows a nearly complete characterization of these plastic pyrolysis oils as GCxGC studies the hydrocarbons present, ICP-OES the elemental balance and FT-ICR MS (with three different ionization modes) unravels the nature of the contaminants.

All pyrolysis oils studied were processed at the same process conditions of 450°C and 1 bar to ensure no dependency on the thermal pyrolysis conditions. GCxGC allowed a full characterization of the hydrocarbon composition. A clear difference between the four pyrolysis oils could be distinguished. The PE pyrolysis oils contain predominantly linear α-olefins and n-paraffins as can be expected due to the linear nature of the PE carbon chain. The PP pyrolysis oil consists of a majority of branched iso-olefins and diolefins due to the branched nature of the PP chain. Furthermore, PP depolymerizes easier as the radical intermediates during thermal dissociation are more stable secondary radicals compared to primary radicals for PE. Additionally, PP contains nearly no aromatics or cyclic compounds as the branched compounds sterically hinder the Diels-Alder reaction which is the common pathway for cyclization reactions. In contrast, MPO contains a significant share of aromatics and naphthenes compared to the separated polyolefins. This as MPO is more contaminated by trace amounts of other polymers such as PS and PET which induce the formation of aromatics.

ICP-OES allowed to determine the predominant heteroatoms present in plastic pyrolysis oils. Oxygen and nitrogen were found to be the most abundant and were present in too excessive concentrations for direct chemical processing in steam cracking. FT-ICR MS allowed a detailed analysis of the nature of these oxygenates and nitrogenates. During pyrolysis, oxygenates are converted to carbon monoxide and carbon dioxide while nitrogen is converted to ammonia. Although the majority of these reaction products leaves the pyrolysis oil, a significant fraction reacts with the hydrocarbon matrix present. FT-ICR MS highlighted a clear tendency of these gasses to react with the aromatic hydrocarbons present. The formation of the more stable aromatic carbon-nitrogen and carbon-oxygen bond favors these reactions over the formation of an aliphatic carbon-nitrogen or carbon-oxygen bond. In addition to this, distinct plastic additives and their reaction products under thermal pyrolysis could be identified. These plastic additives were typically both dominant in the post-consumer waste as in the virgin LDPE pyrolysis oils. Different illustrative examples will be discussed among which DEHP plasticizer, Irganox 1010 antioxidant, and different benzotriazoles used as UV-stabilizers.

Chlorine is one of the most detrimental heteroatoms that are present within plastic pyrolysis oils. This chlorine stems from ill-sorted PVC, additives such as flame retardants or antioxidants, the polymerization Ziegler-Natta catalyst and salts accumulated during consumption. During pyrolysis, the chlorinated organics are converted to HCl which is extremely corrosive for the processing equipment, damaging the downstream infrastructure. Furthermore, chlorine is a potent poison of catalysts and should therefore be limited at all costs. Nevertheless, 137–474 ppm of Cl is measured in post-consumer pyrolysis oils while the industrial limit for steam cracking is fixed at 3 ppm. FT-ICR MS indicated that the chlorine present in the pyrolysis oils was limited, and could be mainly ascribed to dedicated sources such as antioxidants. Nearly no reaction products of secondary reactions with HCl or other chlorinated hydrocarbons were observed. Furthermore, no intermediate products of PVC were found indicating a nearly full conversion to HCl, which is also confirmed by the major difference in chlorine concentration between the solid plastics and their pyrolysis oils (~2000 ppm vs. ~140 ppm). In addition to this, chlorine from the Ziegler-Natta catalysts was also found in combination with titanium and magnesium. No other significant fraction of halogens were found by either ICP-OES or FT-ICR MS.

Metals are a problematic constituent of feedstocks in the (petro)chemical industry. These metals promote catalytic coking in steam cracking and can permanently poison any downstream catalysts. Furthermore, the metals can induce fouling or blocking of processing equipment. Different decontamination strategies exist such as desalination and membrane filtration, of which the latter has already been proven beneficial for pyrolysis oil decontamination [10]. To design these decontamination strategies it is essential to know whether these metals are present as salts or organometallic complexes. FT-ICR MS detected different metalorganic complexes containing Na, Ca, Mg, K, Ti and Cr. These metals stem from additives such as CaCO₃ fillers, sodium phosphate nucleating agents, and the Ziegler-Natta catalysts. In addition to additives, different metals are accumulated by absorption of food or cosmetics by the plastic packaging.

In conclusion, a detailed full-characterization of four plastic pyrolysis oils was obtained by the combination of GCxGC, ICP-OES, and FT-ICR MS. This sheds some light on the origin of plastic waste contaminants and the reactions occurring during thermal pyrolysis. Especially the formation of aromatics has been indicated to be an important factor determining the nature of the pyrolysis oils and its contaminants. The obtained fundamental knowledge will be crucial for designing the necessary decontamination strategies and a sustainable circular plastic economy.

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