Insight into asphaltene transformation during coal tar hydrotreatment by conventional analysis and high-resolution Fourier transform mass spectrometry coupled with collision-induced dissociation technology

Abstract

Asphaltenes are the heaviest and least understood fraction for the petroleum and coal-derived oil resulting in the great challenge during the hydrotreating (HDT) process. In this study, the evolution law of polar functional groups, heteroatom species and structure attributes of coal tar asphaltene was systematically studied at different temperatures using the multifunctional hydrogenation and cracking catalysts through FT-IR, XPS, 1H NMR, 13C NMR and ESI FT-ICR MS coupled with collision-induced dissociation (CID) technology. Most of S has been removed, whereas some refractory O and N are reserved in HDT asphaltenes. The temperature has a significant effect on the low reactivity heteroatom compounds, and high temperature significantly contributes to the hydrodeoxygenation reactions. The OH–OH and OH-ether O are the main hydrogen bonds in both the primary and HDT asphaltenes. The OH-π and cyclic-OH hydrogen bonds decrease with the temperature increasing, whereas that of OH–OH hydrogen bond decreases. The aromatic ether groups increase while those of CO of COOH, C–O of phenols and C–O–C groups decrease after HDT. Both the primary and HDT asphaltenes are dominant by island-type structures and the proportion of island-type fragments increases after HDT. The molecules with multiple O or N atoms are more likely to be an island-type structure in HDT asphaltenes. Besides, the heterolysis mode is the major bond cleavages path for B-NnC and B-NnOxC molecules. The heterolysis mode is the main bond cleavages path for the N-NnC, N-NnOxC and OxC molecules with less heteroatoms in HDT asphaltenes, whereas the homolytic mode dominate for poly-heteroatom molecules. Moreover, there are some “atypical” asphaltene molecules with small size and low aromaticity in both the primary and HDT asphaltenes. The “atypical” asphaltenes are dominant in the removed O1 to O5+ classes.