Molecular Characterization of Aged Bitumen with Selective and Nonselective Ionization Methods by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry. 1. Multiple Pressure Aging Vessel Aging Series

Bitumen is a very complex mixture corresponding to the residue from the vacuum distillation of crude oil. The complexity of such a mixture can be defined by its composition, with several thousand molecules that may contain heteroelements such as sulfur, nitrogen, and oxygen. Almost all bitumen production is used as a binder to produce asphalt pavements. Additional additives or physicochemical treatments can be used to improve the properties and lifetime of this binder. Asphalt pavements are subject to oxidative aging over the years, which leads to deterioration of the physical properties and reduces the performance and durability of the product. To have a better understanding of such mechanisms, artificial techniques can be used to simulate oxidative aging of bitumen. The most-used technique is the pressure aging vessel (PAV) which simulates the long-term aging of bitumens. To characterize oxidative aging at the molecular level, instruments such as Fourier transform ion cyclotron resonance (FTICR) are required. The ultrahigh resolution of this analyzer allows the unambiguous attribution of molecular formulas to each signal. Electrospray ionization (ESI) source was used to selectively ionize acid molecules, while atmospheric pressure photoionization (APPI) source was used to have a nonselective ionization of low-polarity molecules from bitumen. Analyses were carried out on four different PAV aging times (PAV-0h, 25h, 72h, and 96h) to characterize the aging kinetics from a bitumen sample. ESI results allowed the revealing of significant differences from the heteroatom class and family distributions starting from PAV-72h. It was defined by a general decrease of nonoxygenated families (N-x and NxSy) and by an increase of oxygenated families (O-z, NxOz, OzSy, and NxOzSy). The van Krevelen plot evidenced that aging markers correspond to low double bond equivalent (DBE) molecules. The occurrence of aldehydes and fatty acids (DBE 1) with aging in the CHOz family was characterized with DBE versus carbon number plots. Aging markers from OzSy were also characterized with occurrence of benzothiophenic (O2S1) and sulfonic acids (O3S1). Logically, less meaningful results were obtained with APPI. However, this nonselective source allowed the ionization of thiophenes, furans, cycloalkanes, and polycyclic aromatic hydrocarbons that were not observed in ESI measurements. Homogenic decreases in the distribution of nonoxygenated compound classes (HC, S-1, S-2, and N-1) were observed. Concerning the oxygenated molecules, an expansion of the distribution was observed, but without the occurrence of specific aging markers. Oxidation seemed to be favored on linear and naphthenic molecules and not on the highly aromatic ones.

Automated summary

Bitumen, a complex mixture derived from crude oil, is primarily used as a binder in asphalt production. Over time, asphalt pavements undergo oxidative aging, leading to decreased performance. By simulating oxidative aging, researchers can understand the molecular-level oxidation of bitumen.