Boron-Induced Surface Defects in Petcoke as Active Centers for Aerobic Oxidative Desulfurization

Use of metal-free carbonaceous catalysts for efficient sulfur (S) removal from fuel through oxidative desulfurization (ODS), especially using stable oxygen (O2), is limited and challenging as a result of their low O2 activation efficiency. Instead, a considerable amount of research in this area was dedicated to less green peroxide-assisted oxidation as a result of their easier activation than O2 by various metal/metal-free catalysts. In this study, we have synthesized a novel carbonaceous metal-free catalyst from petcoke (PC), a waste from the oil refinery process. The surface of PC was modified by doping heteroatom boron (B) through a solvent-free mechanochemical approach. B-doping-induced defects were hypothesized to generate electron redistribution over the adjacent carbon atoms in PC and provide active sites for the O2 activation to O2• – during the aerobic ODS of the refractory sulfur compound dibenzothiophene (DBT). It was shown that the B loading in petcoke measured by inductively coupled plasma optical emission spectroscopy positively correlated to the DBT conversion as well as the defect concentration as measured by Raman spectroscopy. The active species were identified to be the sites of B–C, B–O, and BCO2 atoms using X-ray photoelectron spectroscopy. Treatment of B-doped PC at 900 °C led to increased B–C interaction and defect concentration, producing 68% DBT conversion compared to 28% at 600 °C treatment, under reaction conditions of 110 °C and 3 h. A further increase in the DBT conversion to 96% was observed at 110 °C and 5 h for the 900 °C treated catalyst. Nearly 42% DBT conversion was observed at 120 °C and 3 h as a result of autoxidation (under no catalyst), which has not been considered in many of the earlier metal-free catalyzed DBT ODS studies. The kinetic analysis suggested a pseudo-first-order reaction for the DBT ODS.