Deuterium exchange of benzo[b]thiophene: Model complex and heterogeneous reactor studies

The complexes [CpRu(BT)]⁺ and [CpRu(3-MeBT)]⁺, where BT is benzo[b]thiophene and 3-MeBT is its 3-methyl derivative, are models for π-coordination of BT to hydrodesulfurization (HDS) catalysts via the benzene ring. The BT in these complexes undergoes base-catalyzed deuterium exchange, at the 2 and 7 positions; in KOH CD₃OD solutions, the exchange follows the rate law: rate = k[Ru complex][OH^-]. The mechanism is proposed to involve rate-determining proton abstraction by OH^- from either [CpRu(BT)]⁺ or [CpRu(3-MeBT)]⁺ followed by transfer of D⁺ from the CD₃OD solvent to give the deuterated BT complex. Heterogeneous reactor studies of deuterium exchange of BT with D₂ over several HDS catalysts (PbMo_6.2S₈, Co_0.25MoS, and 5% Re γ-Al₂O₃) and γ-Al₂O₃ indicate that BT is activated to exchange at the 2 and 3 positions over the catalysts. These results suggest that deuterium exchange of BT over HDS catalysts may occur through routes other than those involving BT π-bonding through its benzene ring to the catalyst surface.