(Zn/Co)-ZIFs@TiO2 composite catalysts for oxidative desulfurization: Impacts of Zn2+/Co2+ on TiO2 interactions

Zeolitic imidazolate frameworks (ZIFs)-based catalysts with a high surface area and structural features have received great attention for potential use in complete eliminating refractory thiophenes from fuels. In this study, new composite frameworks of Zn/Co-based ZIFs@TiO2 were investigated for catalytic oxidative desulfurization (ODS) of dibenzothiophene (DBT). The effect of the type of metal centers (Zn2+ or Co2+) in the ZIF-8 and ZIF-67 frameworks on the coordination with TiO2 and eventually on the catalytic performance was targeted to be studied. The results verified that the intensity of effective N-Ti-O bond was influenced by the size of ZIF crystals and the type of metal ions. Over the ZIF-8-M with small size, the aggregation of TiO2 layer decreased the intensity of N-Ti-O bonds. Meanwhile, the lower energy of Zn-N bond and controlled particle size of ZIF-8-W were favorable for partially replacing Zn2+ with Ti4+ and forming extra N-Ti-O species which are supposed to enrich active Ti-peroxo intermediates for the ODS progress. In the case of ZIF-67, unsaturated Co-N bonds further improved DBT uptake. The maximum ODS efficiency of 98.6% was achieved using ZIF-8-W@TiO2 due to the high surface area (SBET=1548 m2/g), uniform TiO2 distribution, and further N-Ti-O bonds. A good recyclability was also verified for the proposed composite catalyst.