Hydrodesulfurization of dibenzothiophene using Pd-promoted Co–Mo/Al2O3 and Ni–Mo/Al2O3 catalysts coupled with ionic liquids at ambient operating conditions

Abstract

Sulfur compounds in fuel oils are a major source of atmospheric pollution. This study is focused on the hydrodesulfurization (HDS) of dibenzothiophene (DBT) via the coupled application of 0.5 wt% Pd-loaded Co–Mo/Al₂O₃ and Ni–Mo/Al₂O₃ catalysts with ionic liquids (ILs) at ambient temperature (120 °C) and pressure (1 MPa H₂). The enhanced HDS activity of the solid catalysts coupled with [BMIM]BF₄, [(CH₃)₄N]Cl, [EMIM]AlCl₄, and [(n-C₈H₁₇)(C₄H₉)₃P]Br was credited to the synergism between hydrogenation by the former and extractive desulfurization and better H₂ transport by the latter, which was confirmed by DFT simulation. The Pd-loaded catalysts ranked highest by activity i.e. Pd–Ni–Mo/Al₂O₃ > Pd–Co–Mo/Al₂O₃ > Ni–Mo/Al₂O₃ > Co–Mo/Al₂O₃. With mild experimental conditions of 1 MPa H₂ pressure and 120 °C temperature and an oil : IL ratio of 10 : 3.3, DBT conversion was enhanced from 21% (by blank Ni–Mo/Al₂O₃) to 70% by Pd–Ni–Mo/Al₂O₃ coupled with [(n-C₈H₁₇)(C₄H₉)₃P]Br. The interaction of polarizable delocalized bonds (in DBT) and van der Waals forces influenced the higher solubility in ILs and hence led to higher DBT conversion. The IL was recycled four times with minimal loss of activity. Fresh and spent catalysts were characterized by FESEM, ICP-MS, EDX, XRD, XPS and BET surface area techniques. GC-MS analysis revealed biphenyl as the major HDS product. This study presents a considerable advance to the classical HDS processes in terms of mild operating conditions, cost-effectiveness, and simplified mechanization, and hence can be envisaged as an alternative approach for fuel oil processing.