oa δ2H Evolutions of Naphthalene and Methylnaphthalene from the Thermal Pyrolysates of 1-N-Butyldecalin

The formation of naphthalene and alkylnaphthalenes involves the cleavage of multiple C–H bonds and the dehydrogenation of aliphatic rings, leading to stable hydrogen isotope fractionation. In this study, 1-n-butyldecalin (BD) was thermally pyrolysed under 360 °C/50 MPa, both with and without elemental sulfur, to examine the d2H evolution of the alkylnaphthalenes from the pyrolysates of BD. During the initial stages of BD thermal conversion, compounds with the same carbon skeleton—trans-1-methyldecalin (1-MD), 5-methyltetralin (5-MD), and 1-methylnaphthalene (1-MN)—exhibited a consistent trend in d2H values: d2H1-MD < d2H5-MT < d2H1-MN. However, this trend was disrupted as aromatization progressed. A similar pattern of hydrogen isotope evolution was observed in another group of compounds: trans-decalin (D), tetralin (T), and naphthalene (N). These findings demonstrate that hydrocarbon aromatization initially enriches aromatic hydrocarbon in 2H, resulting in a higher d2H value for hydrocarbons with a greater number of aromatic rings compared to those with fewer rings at low aromatization levels. However, as aromatization progresses beyond a certain threshold, the enrichment of 2H in aromatic hydrocarbon diminishes, even leading to higher aromatic-ring-number hydrocarbons exhibiting lower d2H values than lower-ring-number counterparts.