Photocatalytic Activation Of Molecular Oxygen For Deep Removal Of Heterocyclic Sulfur-containing Compounds From Liquid Fuels

Deep desulfurization of fuel plays a vital role in reducing air pollutants,extending lifetime of combustion engines,and meeting the raw materials requirements of fuel cells.It is an important issue for the petroleum refining industry to develop continuously energy-efficient deep desulfurization techniques from liquid fuels.Conventional hydrodesulfurization process is harsh in operation and consumes a large amount of hydrogen.However,it is still difficult to effectively remove heterocyclic sulfur-containing compounds.The photocatalytic desulfurization is considered to be one of the promising technologies because of its mild reaction conditions,availability of solar energy,no consumption of hydrogen,and efficient removal of heterocyclic sulfur-containing compounds from liquid fuelsIn this work,the main contents were photocatalytic activation of molecular oxygen to remove the heterocyclic sulfur-containing compounds from liquid fuels.Graphene oxide,choline-phosphotungstic acid,bismuth oxychloride,and TiO2-MoS2-RGO(RGO indicates the reduced graphene oxide)were selected as the photocatalysts,molecular oxygen(O2)as the oxidant,and acetonitrile as the extractant.The performances of the removal of sulfur-containing compounds from liquid fuels were investigated under ultraviolet(UV)light irradiation Furthermore,the mechanisms of photocatalytic desulfurization reaction were discussed in depth using density functional theory(DFT)calculations(1)Photocatalytic activation of molecular oxygen was investigated through graphene oxide for deep removal of heterocyclic sulfur-containing compounds from liquid fuels.An extraction coupled photocatalytic oxidative desulfurization system for model oil was successfully developed on the basis of as-prepared graphene oxide,air,formic acid,and acetonitrile(MeCN).Under UV radiation,the main reaction conditions influencing sulfur removal were systematically investigated,including the amount of graphene oxide,the volume ratio of MeCN to model oil,the amount of formic acid,the initial S-concentration,air/N2 bubbling,different sulfur compounds,and fuel composition.The photocatalytic oxidative desulfurization mechanism was investigated using free radical trapping experiments,gas chromatography-mass spectrometry(GC-MS),electron spin-resonance spectroscopy(ESR),and DFT calculations.The results indicated that the reactive oxygen species(mainly HO2·and HO·)mainly originated from the armchair edge and the defect sites of the graphene oxide under UV irradiation.These reactive oxygen species further oxidize the sulfur-containing compounds in the liquid fuels,and the oxidation products are removed by acetonitrile extraction to achieve deep desulfurization from liquid fuels(2)Electron-hole interactions in choline-phosphotungstic acid(Ch3-HPW)were investigated to activate molecular oxygen for removal of heterocyclic sulfur-containing compounds from liquid fuels.A facile extraction coupled photocatalytic oxidative fuel desulfurization system was successfully established on the basis of Ch3-HPW,model oil,air,and MeCN.The main photocatalytic reaction conditions affecting the desulfurization process were systematically investigated,including the amount of Ch3-HPW,the volume ratio of MeCN to model oil,the initial S-concentration,air/N2 bubbling,sulfur compounds,and fuel composition.Compared with the model oil,the desulfurization performance of the current system for straight-run gasoline was slightly reduced.This was mainly due to the competitive side reaction caused by the compositional complexity of real gasoline,which leads to a decrease in the desulfurization rate.Moreover,the photocatalytic desulfurization reaction mechanism was further investigated by free radical trapping experiments,GC-MS,ESR,and DFT calculations.The results indicated that the Ch3-HPW has a relative strong electron-hole interaction,and the molecular oxygen was directly activated to form a more active singlet oxygen(1O2)by the energy transfer of electron-hole interaction.The sulfur-containing compounds reacted with the active oxygen to form highly polar sulfur oxides which were transferred to the extract phase,achieving deep desulfurization from liquid fuels(3)Effects of coexisting facets on removal of heterocyclic sulfur-containing compounds from liquid fuels were investigated in BiOCl singlet-crystalline sheets.BiOCl(BOC-01)with twin plane exposure and BiOCl(BOC-02)with triple crystal plane exposure were prepared by adjusting the pH of the reaction system.HRTEM,SAED,and FESEM indicated that the BOC-01 single sheet was mainly hexahedron and the intersecting surface was rectangular.The estimated ratio of {001} and {110} crystal faces to the total surface were?86.5%and?13.5%,respectively.The BOC-02 single sheet was mainly decahedron and the intersecting surface was an octagonal.The estimated ratios of {001},{110},and {010} crystal faces to the total surface were?71.6%,?17.1%,and?11.3%,respectively.The desulfurization test showed that the desulfurization performance of BOC-01 was slightly better than that of BOC-02.The free radical trapping experiments and ESR spectroscopy indicated that there are many reactive oxygen species in the BOC-01 and BOC-02 photocatalytic systems.Among them,1O2,H2O2,and h+were the main active oxygen species for the oxidation of dibenzothiophene,while HO·and O2-?played a secondary role in the desulfurization process.The DFT calculation further indicated that the photogenerated electrons were mainly distributed on the {110} plane in both BOC-01 and BOC-02,while the photogenerated holes were mainly distributed on the {001}crystal plane.Furthermore,the exciton effect played a key role in the photocatalytic molecular oxygen activation in BOC-01 with two co-exposed facets,while the active oxygen species were mainly produced by charge transfer in BOC-02 with three co-exposed facets(4)The electronic structure of TiO2-MoS2-RGO composite and its removal of heterocyclic sulfur-containing compounds from liquid fuels were investigated.The ternary nanocomposite TiO2-MoS2-RGO was prepared by a two-step hydrothermal method.Raman spectroscopy,TEM,XPS,and DFT analysis indicated that TiO2-MoS2-RGO is a "van der Waals heterojunction"formed by weak interaction forces.Weak interaction energy was crucial cause for the stability of the composite.Furthermore,the geometry of TiO2-MoS2-RGO was optimized by DFT calculation,and its electronic structure and charge transfer characteristics were investigated The results suggested the generated electrons are transferred from the valence band of TiO2 to the conduction bands of MoS2 and RGO through multi-point transitions under UV irradiation The extraction and photocatalytic desulfurization reaction system was composed of TiO2-MoS2-RGO,model oil,air,and acetonitrile,and the influences of main reaction conditions on the desulfurization rate were tested.The results indicated that TiO2-MoS2-RGO exhibits enhanced photocatalytic desulfurization performance compared to TiO2,MoS2,and MoS2-RGO This was because the built-in double electric field in the ternary composite effectively reduced the probability of electron-hole recombination and enhanced the photocatalytic activityIn this study,graphene oxide,choline-phosphoric acid,bismuth oxychloride and TiO2-MoS2-RGO were used to activate molecular oxygen to remove heterocyclic sulfides from fuels The desulfurization performances were tested and the reaction mechanisms were systematically investigated.It aims to understand some basic problems of photocatalytic oxidation desulfurization and provide reference for the development of new and efficient clean fuel technology.