Enrichment of nitrogen compounds in heavy oils by complexation with ZrCl4 and detailed characterization using FT-ICR MS and ion mobility spectrometry

•Nitrogen compounds in hydrotreated heavy oils were characterized by FT-ICR MS.•Multi-heteroatomic non-basic nitrogen compounds were enriched by ZrCl4 complexation.•The hydrogenation transformation of multi-heteroatomic nitrogen compounds was revealed.•The structures of refractory basic nitrogen compounds were elucidated by CID and IMS. Nitrogen compounds in heavy oils have significant deleterious impacts on the petroleum products and processes. It is of great significance to characterize the structures of nitrogen compounds during hydrodenitrogenation (HDN) in heavy oils, but the low content of nitrogen compounds after HDN is not conducive to structural analysis. Zirconium tetrachloride (ZrCl4) was found as an efficient complexing agent to enrich both basic and non-basic nitrogen compounds in heavy oils. The molecular weight ranges of the basic and non-basic nitrogen compounds were similar before and after ZrCl4 enrichment. The distribution of carbon number and double bond equivalent (DBE) of basic N1 species in the enriched feedstock was very close to that of the original feedstock, while the basic N1 species in the enriched product behaved a higher DBE distribution. The non-basic nitrogen compounds with higher DBE in the feedstock and product were also more likely to be enriched. Moreover, multi-heteroatomic non-basic nitrogen compounds were more inclined to be enriched than the non-basic N1 species due to the possible synergism of multi-heteroatoms. The numbers of detected multi-heteroatomic non-basic N2, N1O1, and N1O2 compounds obtained by high resolution mass spectrometry increased by 40 to nearly 70 times. The double bond equivalent (DBE) of these refractory multi-heteroatomic non-basic nitrogen compounds in the deep HDN product was much lower than those in the feedstock, due to the hydrogenation saturation reaction during HDN. The representative core structures of refractory basic nitrogen compounds were deduced by ESI+ FT-ICR MS coupled with collision-induced dissociation (CID), and the side chains were proposed by ion mobility spectrometry (IMS). Two typical kinds of refractory basic N1 compounds were suggested in the HDN product. One was a compact structure with multiple naphthenic rings and short side chains. The other had a large molecular size with aromatic rings and long side chains. ZrCl4 as a complexing agent can effectively enrich nitrogen compounds in heavy oil, especially for the hydrotreated heavy oils. This enrichment method facilita