Tracing the composition and structure evolution of oxygen-enriched asphaltenes during the hydrotreating of middle-low temperature coal tar

Abstract

Inefficient conversion of structure-complicated and oxygen-enriched asphaltenes is found as the major challenge facing coal tar in achieving high value-added utilization. Hydrogenation technology is the primary way to achieve the impurity removal and lightweight of coal tar asphaltenes (CTAS). In this study, the evolution laws of functional groups and structural parameters at different reaction times, and pyrolysis behavior at different temperatures of coal tar asphaltenes (CTAS) during the hydrotreating (HDT) process were investigated using the methods of FT-IR, XPS, 1H NMR, 13C NMR and Py-GC/MS. It was found that the increase of the reaction time significantly affected CTAS hydrodeoxygenation (HDO), and most of nitrogen-containing compounds (NCs) and sulfur-containing compounds (SCs) could be removed in a shorter reaction time. However, the increase of the reaction time has a limited effect on the removal of NCs and SCs embedded in polycyclic aromatic compounds. The average alkyl chain length (L) decreased with the increase of the reaction time, and the degree of branching for alkyl side chains (B) increased after HDT. The aromatic ring substitution degree (σ) and alkyl substituents number (n) of CTAS increased after HDT. The OH-OH and OH-ether O were found as the major hydrogen bonds in asphaltene feed (Asp-feed) and asphaltene product (Asp-product), followed by OH-π and cyclic OH. With the increase of reaction time, the molar fraction of the OH-ether O hydrogen bonds increased, while OH-π hydrogen bonds decreased. C-O group was found as the critical oxygen-containing functional group in Asp-feed and Asp-product, primarily existing in the form of aromatic ether groups and C-O of phenols. With the increase of reaction time, the molar fraction of aromatic ether groups in CTAS increased significantly, while that of the C-O of phenols decreased. The pyrolysis fragments of Asp-feed and Asp-product were largely 1–3 ring aromatic hydrocarbons or heteroatoms compounds. Heteroatoms compounds primarily consisted of phenols and furans, whose alkyl side chains were short and few, mainly existing in the form of multiple methyl groups. The Asp-product produced smaller aromatic ring fragments, more aromatic hydrocarbons and less phenolic and non-phenolic oxygenated compounds than Asp-feed at the same pyrolysis temperatures