Structural Analysis of Neutral Nitrogen Compounds Refractory to the Hydrodenitrogenation Process of Heavy Oil Fractions by High-Resolution Tandem Mass Spectrometry and Ion Mobility–Mass Spectrometry

Abstract

This study aims to identify and characterize compounds refractory to hydrodenitrogenation (HDN). The efficiency of a vacuum hydrocracking unit in removing nitrogen-containing compounds to produce a low-nitrogen-content effluent is examined. Molecular, structural, and compositional knowledge is a requisite for the optimization development of the hydrotreatment step processing unit because changes in the chemical composition of petroleum have a direct impact on physical properties and, thus, overall vacuum gas oil (VGO) upgrading processes. Two samples, a VGO and a effluent obtained after the HDN process containing 10 wppm of N, were analyzed by negative-mode electrospray ionization ultra-high-resolution tandem mass spectrometry (ESI–FT-ICR) and ion mobility spectrometry–mass spectrometry (IMS–MS). FT-ICR mass spectrometry provides ultrahigh mass resolving power to separate and characterize compounds in the highly complex petroleum samples. The fragments generated by MS/MS of selected N₁ refractory compounds were used for structural elucidation. Species with a double bond equivalent of 10 and 13, which proved to be highly refractory, were analyzed in more detail. On the other hand, the TWIMS–TOF MS measurements enabled the entire ion mobility analysis of refractory species to hydrotreatment processes. The MS/MS spectra revealed a specific pattern allowing for the identification of the molecular nucleus. Furthermore, it allowed for the understanding of the fragmentation pathways: loss of alkyl chains in the first step and opening and rearrangement of the nuclei after that. Ion mobility separation, in combination with MS/MS, allowed for the identification of the different conformations and the revealment of the typical fragments of these molecular nuclei. In particular, the ion mobility peak width indicates isomeric diversity and collision cross section (CCS) determination and provides structural information. The IMS analysis of the identified refractory precursor shows the evolution of its compounds at different processing stages and indicates that some families and structural conformations of N₁ species are more resistant to hydrotreatment. Isomers presenting low CCS values in negative mode are more resistant to HDN processes. The combination of TWIMS–TOF MS and ultra-high-resolution mass spectrometry opens exciting and promising prospects for structural determination of complex mixtures, in particular, problematic compounds in petroleum refining processes.