Speciation and Semiquantification of Nitrogen-Containing Species in Complex Mixtures: Application to Plastic Pyrolysis Oil

Abstract

Plastic pyrolysis oil is of particular interest for waste management in the current context of a circular economy. Due to their uncontrolled origin, these oils may contain significant amount of unwanted compounds such as nitrogen-containing species. These compounds are known to be catalyst poisons during refining processes. Therefore, the removal of these species is crucial, and their characterization from structural and quantification points of view is essential for this purpose. This study presents a method to specify and quantify nitrogen-containing classes in a plastic pyrolysis oil by direct infusion mass spectrometry. Two steps were used, namely structural characterization to select suitable standards and semiquantification. The structural speciation of nitrogen-containing compounds was first performed by electrospray ionization Fourier transform mass spectrometry, followed by tandem mass spectrometry using high-resolution mass isolation and infrared multiphoton dissociation fragmentation. A semiquantification is then performed by the standard addition method, which is very appropriate for such complex matrices. Aromatic cores such as quinoline and quinoxaline were evidenced for both N₁ and N₂ classes, allowing 2-methylquinoxaline and 2-butylquinoline to be proposed as standards for the semiquantification of N₂- and N₁-containing compounds, respectively. The amount of nitrogen detected from the sum of the individual species was consistent with the bulk analysis. The reported methodology can be applied to numerous other families of compounds in various other complex matrices.

Figure 1

(a) Mass spectrum of the plastic pyrolysis oil obtained via ESI(+). DBE vs carbon number plots for (b) the N₁ family and (c) the N₂ family (stars for polypropylene glycol contaminant).

Figure 2

Fragmentation mass spectrum obtained via IRMPD for the C₁₅H₂₁N₂⁺ ion at m/z 229.169925 attributed to (a) N₂-containing fragments and (b) N₁-containing fragments. Carbon number distribution of (c) N₂-containing fragments and (d) N₁-containing fragments. Putative cores structures for (e) N₂-containing fragments and (f) N₁-containing fragments.

Figure 3

Standard chemical structures of (a) 2-methylquinoxaline and (b) 2-butylquinoline

Figure 4

(a) Methodology of the SAM and its application to plastic pyrolysis oil. (b) Mass spectrum of plastic pyrolysis oil obtained via ESI(+)-FTICR, with enlarged insets showing (c) the 2-methylquinoxaline standard addition and its intensity versus concentration plot and (d) the 2-butylquinoline standard addition and its intensity versus concentration plot.

Figure 5

Plot of DBE vs the carbon number and the DBE distribution as a function of C_mass in solution (ppm N) for (a) N₁-containing species and (b) N₂-containing species. The color map represents the mass concentration in solution (ppm N) of each ion observed in both classes.