The recycling of waste tire has received extensive attention as the number of waste tires increases significantly. It was estimated that around 1.3–1.5 billion pieces of waste tires were produced annually all over the world [1,2]. In 2018, 32 European countries (EU28, Norway, Serbia, Switzerland and Turkey) produced about 3.5 million tons of waste tires and 91% of them were collected and treated [3]. In 2019, China produced about 330 million pieces of waste tires (equivalent to more than 10 million tons) and reused 200 million pieces [4]. In 2020, Canada collected 446,647 tons of waste tires and recycled 97% of them [5]. Waste tires are resistant to heat, biodegradation and mechanical damage. They also take up numerous space due to the voluminous shapes. The reuse of waste tires includes retreading, producing rubber powder and pyrolysis [6]. Waste tires are reused at the physical level by retreading and producing rubber powders. On the other side, pyrolysis is mainly a chemical recycling method which can transform the waste tires into gas, oil and carbon black. As a promising solid waste treatment method, pyrolysis has been widely concerned for recycling the waste tires to fuels and chemical raw materials.

Waste tire pyrolysis oil (WTPO) is an important product of pyrolysis, which is considered as an alternative to fossil fuel [7,8]. Moreover, WTPO is rich in benzene, toluene, xylene and limonene, which are valuable chemicals in industry [8]. The detailed compositions of WTPOs have been widely studied in the past researches [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. The hydrocarbons in WTPOs have been characterized by Fourier transform infrared spectroscopy (FT-IR), gas chromatography-mass spectrometry (GC–MS), high resolution mass spectrometry such as Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), and nuclear magnetic resonance (NMR). The hydrocarbons in WTPOs are mainly attributed to the thermal cracking products of natural rubber and synthetic rubber [1,6,8].

There are also abundant heteroatom compounds in WTPOs, including sulfur compounds and nitrogen compounds. Generally speaking, the main source of the heteroatom compounds are the additives used in tire manufacturing. These additives consist of (i) vulcanization accelerators, such as sulfur, thioureas, 2-mercaptobenzothiazole and N,N’-caprolactam disulphide [19,20], (ii) antioxidants, such as diphenylamine derivatives [21,22], and (iii) plasticizers & softeners, such as fatty acids [23,24]. Typically, about 6 wt% of vulcanization accelerators and plasticizers are added in the tire formulations, and up to 2 wt% of antioxidants and other chemicals are also used [25]. Another source of the heteroatom compounds is the tire cord fiber, which is mainly consist of poly(ethylene terephthalate) (PET) [26,27]. The heteroatom compounds are undesirable components in WTPOs, which are harmful to the combustion behavior and environment. A better understanding of the structures of the heteroatom compounds would be helpful to the removal of these compounds.

The structures of sulfur compounds in WTPOs are mainly thiophenes, benzothiophenes, dibenzothiophenes and naphthothiophenes. Williams et al. found that the contents of sulfur compounds in WTPOs increased with the pyrolysis temperature, which was due to the enhanced aromatization reaction between polyaromatic hydrocarbons and sulfur at higher temperature [28]. Liu et al. investigated the pyrolysis mechanisms of rubbers and corresponding vulcanized rubbers, and found that the formation of sulfur compounds during the pyrolysis of the vulcanized rubbers would also affect the chain scission reactions of the rubbers [29].

The types of nitrogen compounds in WTPOs are more complex, including benzothiazoles, pyrroles, indoles, anilines, diphenylamines, pyridines, quinolines, amides and nitriles. Mirmiran et al. separated the nitrogen compounds from the naphtha fraction of a WTPO and detected over 30 nitrogen compounds by GC–MS, such as caprolactam, picoline, pyridine, aniline, pentanedinitrile and benzothiazole [20]. Pakdel et al. characterized the naphtha fraction of a WTPO directly by GC-FTIR-MS, and found several nitrogen compounds such as acetonitrile, propanenitrile, isobutanenitrile, pentenenitrile, hexanenitrile, indole, pyridine, aniline, benzothiazole and pyrazole derivatives [9]. Laresgoiti et al. characterized the liquid pyrolysis products of tire by GC–MS, and found benzothiazole, benzonitrile and ethylquinoline [12]. Islam et al. reported several nitrogen compounds containing multi-heteroatoms in the bicycle/rickshaw tire pyrolysis oil, such as ethinamate (C9H13NO2) and 5-methyl-6-phenyltetrahydro-1,3-oxazine-2-thione (C11H13NOS) [13]. Frigo et al. detected relatively high amount of benzothiazole in the light fraction of WTPO [14]. Campuzano et al. measured the tire pyrolysis oils by atmospheric pressure photoionization (APPI) FT-ICR MS, and significantly higher abundance of N2 species was found compared to N1 species [18,19]. Campuzano et al. also characterized the light fraction of tire pyrolysis oil by GC–MS and detected benzothiazole, 2,4-dimethylquinoline and N-(1,3-dimethylbutyl)-N'-phenyl-1,4- benzenediamine [19]. Palos et al. studied the nitrogen compounds in the scrap tires and its hydrodenitrogenation products by electrospray ionization (ESI) FT-ICR MS, and the results indicated that carbazoles, indoles and quinolines with lighter and less aromatic rings were more resistant during hydrodenitrogenation [30].

In addition to these nitrogen compounds, aliphatic nitriles were also widely found in WTPOs. Gu et al. enriched the polar fraction of WTPO by column chromatography, and detected hexadecanenitrile and heptadecanenitrile by GC–MS [31]. Tudu et al. found benzonitrile, hexadecanenitrile, heptadecanenitrile and octadecanenitrile in the light fractions of tire pyrolysis oil by GC–MS, which was quite different from the common diesel oil produced by fossil fuels [32]. Ayanoğlu et al. also found benzonitrile, 2-methyl-benzonitrile, hexadecanenitrile and octadecanenitrile in the light fraction of WTPO [33]. Chen et al. also detected butyronitrile and pyrrolidinedicarbonitrile in waste tire pyrolytic oils by GC–MS [34].

The origin of some nitrogen compounds in WTPOs can be easily interpreted. For example, benzothiazole and caprolactam are mainly from the thermal cracking of vulcanization accelerators (2-mercaptobenzothiazole and N,N’-caprolactam disulphide), anilines may be derived from the pyrolysis of vulcanization accelerators (diphenylguanidine derivatives) or antioxidants (diphenylamine derivatives), and N-(1,3-dimethylbutyl)-N'-phenyl-1,4-benzenediamine is a commercial antioxidant (antioxidant 4020) which is widely used in the tire formulations.

However, the detailed formation process of benzonitrile and aliphatic nitriles in WTPOs is still not clear to the best of our knowledge. Although aliphatic nitriles were mentioned as rubber softeners in the very early stage [35], they have not been widely applied in rubber industry. The rubber plasticizers & softeners used in industry are based on mineral oils, animal and plant oils, and synthetic oils [36]. Among all of the synthetic oils used as rubber softeners, liquid acrylonitrile-butadiene rubber is the only one that contains nitrile groups, but it's seldom used in tire formulations. Moreover, the main pyrolysis products of acrylonitrile-butadiene rubber are acrylonitrile (C3H3N), 1-cyano-2,4-pentadiene (C6H7N) and 1-cyano-2,4-hexadiene (C7H9N) [37,38], which are quite different from the stearonitriles found in WTPOs [31], [32], [33]. Benzonitrile and aliphatic nitriles were suspected as by-products of benzothiazoles and diphenylamines during the pyrolysis of waste tires [39]. However, the detailed reaction routes were not suggested. As a result, the origin of the nitriles in WTPO is still uncertain due to the lack of intermediate products.

The focus of this study is to find the intermediate nitrogen products during the pyrolysis of waste tires and explain the formation of the nitriles. In order to obtain more structural information of nitrogen compounds in WTPOs, a series of separation and enrichment treatments were carried out. First, the WTPO was distilled into four parts according to the boiling points, which were initial boiling point (IBP) to 150 °C, 150–180 °C, 180–350 °C and higher than 350 °C. Then, the nitrogen compounds in the light fractions with boiling points below 350 °C were separated by solid phase extraction (SPE) and analyzed qualitatively by GC–MS. These nitrogen compounds in the unfractionated light fractions were then quantitatively measured using GC–NCD. On the other hand, the nitrogen compounds in the heavy pyrolysis oil (above 350 °C) were enriched according to the differences of solubility and polarity. The structures of the enriched nitrogen compounds were further studied by tandem mass spectra obtained by FT-ICR MS, which were mainly in the form of amide derivatives of diphenylamine. These amide derivatives in the heavy pyrolysis oil were supposed as intermediate products of nitriles during the pyrolysis of waste tires.