Remarkable Fractionation of the Hydrogen Isotope (δD) of Hydrocarbon during the Fast Aromatization Process: Insights from the Pyrolysis Experiments of n-Butylcyclohexane

Hydrocarbon aromatization involves the cleavage of multiple C–H bonds and the dehydrogenation of aliphatic rings, with a resultant fractionation of stable hydrogen between aromatic hydrocarbons and their counterpart precursors. Despite this understanding, there is still the paucity of studies on the hydrogen isotope (δD) fractionation of aromatic compounds and the δD fractionation characteristics during the aromatization process of hydrocarbons. In the present study, n -butylcyclohexane (BCH) was thermally pyrolyzed in a closed gold tube system at different time intervals to investigate the aromatization process of the monoaliphatic ring hydrocarbon. The δD evolutions of toluene (Tol) and methylcyclohexane (MCH) from the BCH pyrolysates were also examined, with the aim of studying the cleavage behavior of the C–H bond at the aliphatic ring. The results of the 24 h BCH pyrolysates showed that toluene was strongly enriched with 2 H, whereas MCH was heavily depleted, with ΔδD Tol–MCH (δD Tol – δD MCH ) being 162.3‰ [Vienna Standard Mean Ocean Water (VSMOW)]. In comparison to the initial hydrogen isotope of BCH, ΔδD MCH–BCH and ΔδD Tol–BCH were −46.5 and 115.8‰, respectively. With an increasing pyrolysis time, the degree of δD fractionation between Tol and MCH gradually mitigated. It is speculated that the dehydrogenation of the aliphatic ring started with more possibility of cracking the C– 1 H bond and then having more C– 2 H being retained in the aromatic ring at the initial stage of hydrocarbon aromatization, leading to the strong δD fractionation of the aromatic hydrocarbon. This observation is quite instructive, in that fast aromatization of the hydrocarbon caused a remarkable hydrogen isotope fractionation, which may play a significant role in the strong 2 H enrichment of aromatic hydrocarbons generated from a fast and high-temperature thermal event, such as a meteorite when passes through the atmosphere.