Studies of the mechanism of thiophene hydrodesulfurization: ²H NMR and mass spectral analysis of 1,3-butadiene produced in the deuterodesulfurization (DDS) of thiophene over PbMo₆S₈ catalyst

The deuterodesulfurization (DDS) of thiophene was investigated over PbMo₆S₈ at 400°C using a flow-microreactor. Evidence indicates that 1,3-butadiene (BDE) is the first desulfurized product; its deuterium content was established by ²H NMR and mass spectrometries. At different levels of thiophene conversion (0.86-10.2%), the amount of deuterium incorporated into BDE remains constant at 3.47 D atoms per BDE molecule. Unconverted thiophene incorporates 0.42 D atoms at 10.2% thiophene conversion but only 0.05 D atoms at 0.86% conversion. Reaction of 2,5-dihydrothiophene (2,5-DHT) with D₂ at 400°C over PbMo₆S₈ liberates BDE as the only hydrocarbon product. This BDE incorporates no deuterium. Thiophene and H₂S effectively inhibit both BDE hydrogenation and deuterium exchange. The results indicate that during the DDS process, a total of 3.2 deuterium atoms are incorporated into the BDE; 0.83 D are in the D_A-position while 1.2 D are in each of the D_B-and D_C-positions. Several HDS mechanisms proposed in the literature are consistent with these results; two are not. Details of all of these mechanisms are discussed.