Encapsulation of ionic liquid, phosphotungstic acid inside the nanocages of MIL-101(Cr): Effective and reusable catalyst for efficient solvent-free oxidative desulfurization from fuel oil

Oxidative desulfurization from fuel oil is one of the important methods for deep desulfurization. The development of efficient oxidative desulfurization catalysts is crucial for improving the desulfurization performance. Successful encapsulation of phosphotungstic acid (HPW) and ionic liquid (BMImBr) inside the mesoporous cages of MIL-101(Cr) was accomplished through a combination of “bottle around ship” and “ship in bottle” methods. The obtained BMImPW@MIL-101(Cr) composite was characterized by XRD, FTIR, BET, SEM, XPS and ICP methods. Results indicated that the BMImPW@MIL-101(Cr) composites with PW³⁻ loading of 23.1–50.7 wt% were obtained, demonstrating that the “bottle around ship” method is beneficial to make full use of nanocages of MIL-101(Cr) to obtain expected high loading of active PW³⁻ . The BMImPW@MIL-101(Cr) exhibits excellent reusability with no evidence of leaching of active PW³⁻ and BMIm⁺, and well-preserved structure after successive cycles of regeneration and reuse. The significantly improved stability of BMImPW@MIL-101(Cr) as compared to HPW@MIL-101(Cr) is possibly because the leaching of the active PW³⁻⁻ sites can be greatly suppressed by forming large size of BMImPW owing to introduction of BMIm⁺ cation. The BMImPW@MIL-101(Cr) exhibited excellent catalytic activity for solvent free oxidative desulfurization of refractory sulfides. The enhanced oxidative desulfurization activity as compared to HPW@MIL-101(Cr) can be explained by the intimate contact of sulfides with active PW³⁻ sites owing the strong attraction of BMIm⁺ cation with the sulfides.