Hydrogen isotope fractionation during aromatization to form alkylnaphthalene: Insights from pyrolysis experiments of 1-n-butyldecalin

Alkylnaphthalene homologues are important components of aromatic fraction in sedimentary organic matter and contain significantly geochemical information relative to formation and evolution of the host organic matter. They mainly originate from hydrocarbon aromatization reaction which involves the dehydrogenation of aliphatic rings resulting in the fractionation of stable hydrogen isotopes between aromatic hydrocarbons and their precursors. To examine these processes, this study thermally pyrolysed 1-n-butyldecalin (BD) at different time intervals under 360 °C/50 MPa to study the aromatization and hydrogen isotope fractionation during alkylnaphthalene formation and evolution. The relative content of aromatic products, such as naphthalene (N) and 1-methylnaphthalene (1-MN), increases with increasing aromatization. Sulfur enhanced the degree of aromatization during BD thermal evolution, resulting in greater N and 1-MN formation. For the compounds with the same carbon skeleton, i.e. tran-1-methyldecalin (1-MD), 5-methyltetraline (5-MT) and 1-MN, the ²H enrichment follows the order δ²H_1-MD < δ²H_5-MT < δ²H_1-MN during the low thermal conversion of BD. However, the order was subsequently destroyed with increasing aromatization. The results indicate that hydrocarbon aromatization can enrich aromatic hydrocarbon in ²H, resulting in a higher δ²H value of higher aromatic-ring-number hydrocarbon than that of a lower aromatic-ring-number at low aromatization. However, ²H enrichment will decrease and even result in a reverse order with enhanced aromatization. Our findings are beneficial for understanding genetic mechanism and hydrogen isotope fractionation effect during the formation and evolution of aromatic hydrocarbons.