Desulfurization mechanism of thiophene compounds in supercritical water

Thiophene and its derivatives, the major sulfur-containing compounds widely exist in crude oil, which will cause air pollution by releasing SOx during direct combustion. Hence, the desulfurization of thiophene compounds was essential. Supercritical water (SCW) treatment is a potential technique for sulfur removal and energy conversion. In this work, density functional theory (DFT) calculations and experiments are employed to study the decomposition of thiophene compounds in SCW. DFT shows that H2O plays the role of reactant as well as catalyst in the delocalization destruction of thiophene. Additionally, H2, the main gaseous product in SCW process, also promotes the decomposition of thiophene. What's more, a larger substituent in thiophene lead to a higher reaction energy barrier for decomposition. Experimental results show that the maximum desulfurization rate of thiophene reaches 91.54% in SCW, which is almost 1.5 times as that in pyrolysis (61.06%) at 700 °C and 30 min. And dibenzothiophene with the largest substituent make the poorest performance in desulfurization compared with methyl-thiophene and benzothiophene, which proved the results of DFT calculations. Based on the above research, the detailed reaction pathways of dibenzothiophene are proposed, which may be critical in understanding the desulfurization of thiophene compounds and the resource utilization of crude oil in SCW.