Deep oxidative desulfurization catalyzed by Ti-based metal-organic frameworks

Abstract

Ti-based metal-organic frameworks (MOFs) MIL-125 and NH2-MIL-125 were synthesized by the solvothermal method and characterized by X-ray powder diffraction (XRD), Fourier transform infrared (FT-IR), scanning electron microscopy (SEM) and diffuse reflectance UV–vis spectroscopy (DR/UV–vis). The crystal sizes of MIL-125 were regulated by changing the concentration of Ti ions in the synthesis system. Catalytic properties of MIL-125 and NH2-MIL-125 were evaluated in extractive and catalytic oxidative desulfurization (ECODS) by using H2O2 as the oxidant. In the ECODS system, the oxidative removal rate using MIL-125 or NH2-MIL-125 increases in the order as 4, 6-dimethyldibenzothiophene (4,6-DMDBT) < thiophene (Th) < benzothiophene (BT) < dibenzothiophene (DBT). The presence of –NH2 group in NH2-MIL-125 prevents sulfur compounds from contacting with Ti sites in a certain extent, resulting in the lower removal rate of NH2-MIL-125 comparing with that of MIL-125 in most cases. MIL-125 with smaller crystal size presents lower sulfides removal rate, which indicates that the active titanium species in MIL-125 is the decisive factor to affecting ODS. Both NH2-MIL-125 and MIL-125 show high stability in the presence of methanol and still maintain the integrity of the structure after being recycled five times. Methanol can inhibit H2O2 from damaging Ti-based MOFs and extract the oxidized products, which is beneficial to the ODS. While in the absence of extractant such as methanol, tert-butyl hydroperoxide (TBHP) is the optimal oxidant.