Unexpected Structural Effects on the Onset of Thermal Reactions of Aromatic Hydrocarbons

Abstract

This work aims to study the reactivity of a broad range of aromatic hydrocarbons to better understand the reactivities of more complex hydrocarbon mixtures. A set of closed-system pyrolysis experiments were conducted on a diverse range of ∼30 polycyclic aromatic hydrocarbons (PAHs) at the thermal onset reaction temperature of 400 °C to understand structural effects on reactivities and shed light on the early events of thermal reaction mechanisms. Thermal transformations, including methyl transfer (alkylation/dealkylation), rearrangement, condensation, molecular growth, and other chemical transformations, are often indicated by dark carbon materials composed of complex mixtures. Thermal transformation was confirmed by phenomenological changes (color and solubility) and chemical analysis using nuclear magnetic resonance, thermogravimetric analysis, gas chromatography, and electron spin resonance. Unsubstituted PAHs (e.g., phenanthrene, anthracene, pyrene, chrysene, etc.) were unaffected by heating at this temperature, with the exception of tetracene and pentacene which readily transformed into dark and insoluble materials. The reactivity of these unsubstituted PAHs is consistent with aromaticity predicted by the Clar theory. On the contrary, most alkyl-substituted PAHs can be transformed into carbon materials at 400 °C; however, alkyl-substituted phenanthrenes (2,7-dimethyl, 2-ethyl, or 7-ethyl) and benzenes (xylenes and pentamethylbenzene) were unreactive. CH2-containing PAH molecules were unreactive if in a five-membered ring (e.g., fluorene and benzofluorene) but became reactive if in a six-membered ring (9,10-dihydroanthracene), likely as a result of aromatization of the latter. The reactivity of the aryl–aryl linkage depends upon the aromatic moieties to which it is connected. While it is unreactive when connecting phenyl groups, such as in p-terphenyl, it is reactive when connecting pyrenes, such as in 1,1′-bispyrene. These results were unexpected and challenge the conventional thinking about hydrocarbon reactivities. Explanations and possible hypotheses are proposed, but more questions remain unanswered to understand the structural effects on reactivities. These findings are conducive to the sustainable use of aromatic hydrocarbons for higher value carbon materials and relevant to numerous pyrolysis studies on oil shale kerogens, biomass, and waste plastics.

Asphaltenes from Ethylene Tar as a Potential Raw Material to Obtain High Value-Added Products

“…It is considered that SP-2 is the product of oxidative condensation of asphaltenes, which becomes insoluble in most solvents. In addition, there was a large decrease in the absorption intensity of the bands at 2858 and 2954 cm −1 corresponding to methyl and methylene groups, which agrees with the formation of crosslinks at cleavage sites of alkyl groups during oxidation of PAHs [58,59]. There is a large increase in the oxygen content in the SP-2 product (Table 4).…”

Accessing the Low-Polar Molecular Composition of Boreal and Arctic Peat-Burning Organic Aerosol via Thermal Analysis and Ultrahigh-Resolution Mass Spectrometry: Structural Motifs and Their Formation

“…Figure a shows the mass loss for all four peat samples under nitrogen and oxidative conditions with additional information on the exact mass loss values and residue content given in Table . Generally, oxidative conditions reveal higher mass losses than smoldering conditions, which can be explained by more efficient combustion of the peat, whereas during oxygen-depleted smoldering, thermally induced condensation and aromatization reactions lead to the formation of coke residue. , NOR has, in general, higher TGA residues because of its higher mineral content. The sampling location of NOR is in the high Arctic areas covered by ice or tundra with short vegetation .…”

Effect of Methyl Groups on Formation of Ordered or Layered Graphitic Materials from Aromatic Molecules

“…[11,12,50,51] Given the existing insight, this work adds to the existing body of literature by contextualizing the methyl-mediated crosslinking beyond a single molecule toward the mesoscale aromatic hydrocarbon systems with direct validation from experiments. [52,53] With a clearer understanding of reaction mechanisms in such systems, a better atomistic modeling framework can be created that can capture chemical phenomena more accurately.…”