Synthesis of silica@C-dots/phosphotungstates core-shell microsphere for effective oxidative-adsorptive desulfurization of dibenzothiophene with less oxidant

A core-shell microsphere catalyst with a solid silica core and a mesoporous shell made up of quaternary ammonium phosphotungstate and carbon dots (C-dots) was successfully synthesized for the oxidative-adsorptive desulfurization (OADS) of dibenzothiophene (DBT) in fuel oil. The phosphotungstic acid used for the synthesis of the catalyst was calcined to enhance its activity. XRD, FT-IR and catalytic test showed that the defective Keggin anion [PW12O38]3− formed under proper calcination conditions could explain the activity enhancement. EPR measurement indicated that hydroxyl radicals were generated from H2O2 by the interaction with C-dots, which proved C-dots to be an efficient cocatalyst in the system. The synergetic effect of [PW12O38]3− and C-dots in the synthesized catalyst remarkably improve the utilization efficiency of H2O2. When the loading amount of phosphotungstate and C-dots were optimized to 25% and 0.45%, the OADS system showed best desulfurization capacity under 50 °C. A high OADS efficiency of 98.08% could still be achieved even when the ratio of n(H2O2)/n(DBT) was reduced to 1.75. The existence of dibenzothiophene sulfoxide (DBTO) in the acetonitrile eluent of spent catalyst was confirmed by LC–MS spectra. The characterization results indicated that part of the DBT was oxidized to its corresponding sulfoxide which need less oxidant than sulfone. Both of the oxidation products could be eliminated from oil phase by adsorption on the catalyst.