Preparation Of Molybdenum-based Catalysts And Their Performance And Mechanism For Deep Oxidative Desulfurization Of Fuels

In the current fuel desulfurization strategy,oxidative desulfurization(ODS)can not only remove refractory aromatic organic sulfur compounds under mild reaction conditions,but also selectively convert sulfur compounds into valuable sulfones or sulfoxide products,which has attracted the attention of many researchers and is expected to replace or strengthen the traditional oil desulfurization technology(hydrodesulfurization).However,to achieve efficient and selective catalytic oxidation of organic sulfur compounds,catalysts and reaction conditions are key factors.Therefore,in this paper,Ti-doped silicate-supported molybdenum trioxide(MoO₃)composite catalysts and electron-rich nitrogen-doped carbon-supported defective molybdenum dioxide(V_O-MoO₂)composite materials were designed and prepared,respectively.The physicochemical properties of the catalysts were analyzed by various characterization techniques combined with density functional theory(DFT)calculation.Meanwhile,the ODS reaction conditions of the catalysts were optimized via experiments,so as to achieve ultra-deep desulfurization of fuel.The main work is as follows:1.Three-dimensional ordered mesoporous bifunctional catalyst with the function of catalyzing the oxidation of organic sulfur and separating the product from the reaction mixture.Here,MoO₃ nanoparticles supported on Ti-doped KIT-6(average particle size=1.33 nm)can be synthesized by P123(EO₂₀PO₇₀EO₂₀)-assisted in-situ synthesis combined with stepwise pyrolysis.The composite catalyst(Mo/KIT-6-Ti)exhibits excellent ODS performance.When using cumene hydroperoxide(CHP)as oxidant,Mo/KIT-6-Ti not only can completely remove dibenzothiophene(DBT)from fuel at room temperature in 40 min,but also has excellent adsorption and separation ability of sulfone from fuel.After five reaction-regeneration cycles,the DBT conversion rate on Mo/KIT-6-Ti still reached more than 99%within 60 min.In addition,the kinetic study indicated that the oxidation reaction of DBT on Mo/KIT-6-Ti exhibited a very low activation energy(34.9 k J/mol).The data of free radical quenching experiments revealed that hydroxyl radicals play a major role in ODS.The catalyst has the potential to eliminate the demand for solvent extraction steps in fuel desulfurization and produce ultra-low sulfur fuel.2.The effect of electron transfer between metal oxides and supports on the ODS activity and stability of the composite catalyst was clarified.In this work,molybdenum dioxide with oxygen vacancies(V_O-MoO₂)catalysts derived from Mo-based organic frameworks were anchored on electron-rich nitrogen-doped carbon nanotubes(NC)to obtain excellent ODS activity and stability.When CHP was used as an oxidant,either DBT or 4,6-dimethyldibenzothiophene(4,6-DMDBT)in the fuel could be 100%removed after catalytic oxidation on the composite catalyst(V_O-MoO₂@NC)for 40 min.After five reaction cycles,the removal rates of DBT and 4,6-DMDBT on V_O-MoO₂@NC still exceeded 99.5%and 95.0%,respectively.The results of DFT calculation and characterizations confirmed that the strong electron donor effect of NC on V_O-MoO₂ could promote the dispersion of V_O-MoO₂ and weaken the bond energy of Mo–O bond,leading to the exposure of active sites and the enrichment of oxygen vacancies(V_O).In addition,the strong electrostatic interaction between interfaces caused by electrons transfer from NC to V_O-MoO₂ can reduce the leaching of active phase of catalyst.This study will provide a reference for the reasonable adjustment of the electronic structure of the catalyst to optimize the performance of ODS,and open up a new idea for the construction of high performance ODS catalysts,that is,by supporting transition metal oxides on NC to enhance the electronic interaction between metal oxide and support.