Highly dispersed titanium-oxo clusters encapsulated in MIL-101(Cr) cages via amidation for efficient oxidative desulfurization of fuel oil

A novel and robust catalyst for oxidative desulfurization (ODS) of fuel oil was fabricated by encapsulating titanium-oxo clusters with amino-groups (Ti₆abz₆, abz denotes 4-aminobenzoic acid) into the mesoporous cages of MIL-101(Cr). This strategy employs an amidation reaction with succinic anhydride that acts as a molecular expander to realize an expansion-trapping process. This approach can effectively prevent clusters aggregation/leaching while maintaining the accessibility of active sites. The optimized catalyst named as 0.4Ti₆abz₆@MIL exhibited superior catalytic performance in the ODS reaction of DBT. Its turnover frequency reached 61.3 h⁻¹ at 60 °C, which is much higher than those of the composites prepared by impregnation and physical-mixing methods. The results from electron paramagnetic resonance indicate that hydroxyl radicals (•OH) are the dominant active species in the oxidation reaction. Furthermore, density functional theory calculations revealed that the introduction of amide linkages significantly strengthens the adsorption of H₂O₂ through enhanced hydrogen-bonding interactions, inducing the elongation of peroxoxide bond and effectively facilitating its cleavage to generate reactive •OH radicals. This work demonstrates that amidation is an effective strategy for encapsulating highly dispersed and stable titanium-oxo clusters into the pores of MOFs.