Oxidative desulfurization of dibenzothiophene over monoclinic VO 2 phase-transition catalysts

Phase-pure monoclinic VO2 (M) was successfully synthesized by hydrothermal reduction method using oxalic acid as a reductant and solvent-thermal reduction method using methanol as a reductant. Oxidative desulfurization (ODS) activities were investigated and the results revealed that intrinsic ODS activity increased with increasing cell volume for different synthesized VO2 (M) catalysts. DSC measurement gives the evidence that phase transition temperature of monoclinic VO2 (M) to rutile VO2 (R) decreased with increasing cell volume for different VO2 (M) catalysts. Variable-temperature in-situ Raman spectrum also confirmed that phase transition from VO2 (M) to VO2 (R) occurred. Mechanism of ODS reaction on VO2 (M) catalyst was investigated precisely involving temperatures which phase transition occurred. Below the phase-transition temperature VO2 (M) showed higher ODS reaction activation energy, while above the phase-transition temperature VO2 (R) exhibited lower ODS activation energy. This result may be explained by the fact that VO bond length for VO2 (R) (1.93 Å) fits more for the five-member ring formed with the oxidant tert-butyl hydroperoxide (TBHP) during ODS reaction compared with V-O bond length for VO2 (M) (about 2.03 or 1.86 Å).