Catalytic performance and stability of (V) MIL-47 and (Ti) MIL-125 in the oxidative desulfurization of heterocyclic aromatic sulfur compounds

A Ti-based metal organic framework (MOF), MIL-125 (MIL stands for Material from Institut Lavoisier), and a V-based MOF, MIL-47, were synthesized and tested for their catalytic activity in the oxidation of three heterocyclic sulfur compounds (dibenzothiophene, benzothiophene, and thiophene). Dibenzothiophene (DBT) oxidation studies were used to determine kinetic parameters, and activation energies for MIL-47 and MIL-125 were found to be 51 kJ/mol and 75 kJ/mol, respectively. Benzothiophene and thiophene oxidation studies were used to show that the greater catalytic activity of MIL-47 was not simply due to the limited accessibility to the active sites of MIL-125 in the pores but rather was also a function of the type of metal center. MIL-125 had faster reaction rates in the oxidation of thiophene than DBT, which indicated that catalysis occurs on the surface of the catalyst as well as in the pores. Without DBT, however, MIL-47 degraded first into MIL-47as and then eventually 1,4-benzenedicarboxylic acid (BDC) and vanadium oxide in the presence of tert-butyl hydroperoxide (TBHP) alone. This degradation occurred even at lower temperatures in the presence of TBHP for 0.5 h. MIL-125, however, was stable in the presence of TBHP even under harsh reaction conditions (24 h at 100 °C).