Synthesis, Modification And Oxidative Desulfurization Catalytic Performance Of Heteropoly Acid/Mesoporous Silica Nanocomposites

Since its report in 1992, the mesoporous material has become the leading edgeand hot issue in the field of chemistry and materials as a result of its broad applicationprospect and scientific connotation. In past two years, the amount of research papersrelated to mesoporous material has run to approximately 2200 each year and theresearch contents mainly focus on the following aspects: (1) research on the synthesisand features of ordered mesoporous material with different elements, skeletonstructure and mesoporous morphography; (2) synthetic method and techniqueoptimization; (3) Functionalization and application of ordered mesoporous material.This paper is to combine the research of mesoporous material with the globalenvironmental deterioration. By designing and synthesizing heteropoly acid with highcatalytic activity/mesoporous silicates composition, the author tries to recombinemesoporous material with heteropoly acid on the basis of the uniqueness ofmesoporous material in macromolecule catalytic reaction and to apply it to theresearch on oxidative desulfurization of macromolecule organic sulfide in diesel fuel.This paper has investigated different mesoporous composite materials as well as theinfluence of their preparation method on catalytic activity and probed into its catalysismechanism. The fruits of this paper are that we have obtained three kinds of newcatalysts with drastic desulfurization of diesel fuel and solved the immobilization ofheteropoly acid. The innovative points of this paper are listed as follows:1. Usingγ-ammonia propyl-triethoxysilicane alkane (APTES) surfacemodification mesoporous silicate SBA-15 and by the fixation of amino, the author hasimmobilized phosphotungstic acid (HPW) and phosphomolybdic acid (HPMo)with Keggin structure on the surface of modification SBA-15 by means of traditionalimmersion method. As result, the triconstitutent nanocomposites with amino moietiesas the transition layer, i.e. HPW/Amino/SBA-15 and HPMo/Amino/SBA-15 wereobtained. By comparing the catalytic oxidation of two kinds of loading heteropolyacid catalyst to dibenzothiophene (DBT) in solution, the author has discovered thatloading HPW catalyst possesses much higher catalytic activity. In theimmobilization process of heteropoly acid, low acidic environment and high refluxingtemperature can result in partial retrogradation of phosphotungstic acid, but itscatalytic activity has no evident change after five circles, due to easy separation andresuage for the loading heteropoly acid. 2. With hexadecyl trimethyl ammonium bromide (CTAB) as template and withethyl orthosilicate (TEOS) and HPW as inorganic source, the author has synthesizedHPW/SiO₂ nanocomposite material with mesostructure in one pot. In comparasion,HPW/SiO₂ composite material with microporous structure was also prepared (referredto HPW/SiO₂(Ami)). By comparing the catalytic oxidation activity of the above twoloading heteropoly acid catalysts with different pore structure and charactarization toDBT solution, the author has also discovered that mesoporous HPW/SiO₂(Ame)nanocomposite catalyst has higher oxidative desulfurization catalytic activity onmacromolecular DBT oxidation, which illustrates that mesostructure plays a key roleon catalytic oxidation of macromolecular DBT. This catalyst has preserved the Kegginstructure of heteropoly acid. The frist percent conversion of DBT was 99.4% withintwo hours, and after seven usages, that could be 80.6%.3. With TEOS and HPW as inorganic source and with triblock copolymerpolyethylene oxide- polypropylene oxide- polyethylene oxide (PEO-PPO-PEO, P123)as template, the author has synthesized mesoporous HPW/SiO₂ nanocompositematerials (referred to HPW/SiO₂(E-x%)) by evaporation-induced self-assemblymethod. The main strategy of this method is to utilize the self-generated acidity ofphosphotungstic acid (HPW) to drive tetraethoxysilane (TEOS) hydrolysis. TheTEOS hydrolysis leads to forming the positive charge silicate species that cancombine with HPW anion to form colloidal Si-HPW entities resulting from the staticinteraction. These Si-HPW entities can be assembled to form mesostructure withsolvent evaporation. This catalyst has preserved the Keggin structure of heteropolyacid and possesses excellent catalytic activity in the catalytic oxidation process ofDBT organic sulfide. The percent conversion of DBT can reach 99.6% within twohours while that has no evident catalytic activity loss after seven periodic duties.4. Based on the research of the catalytic oxidation reaction kinetics of varioustypical organic sulfide single component imitation systems, the author discovers thatthe order of the apparent activation energy of organic sulfide is that dibenzothiophene＞4,6 Dimethyl-dibenzothiophene＞benzothiophene. Probing into the catalyticoxidation reaction mechanism of HPW/SiO₂(E-x%) nanocomposites for typicalorganic sulfide in diesel oil, the author brings to light that its catalytic mechanism ofaction is in consistent with typical Ishii-Venturello catalytic mechanism, i.e., Kegginnegative ion [PW₁₂O₄₀]^3- immobilizing on mesoporous silicate is firstly oxidized asperoxoicpolyacid negative ion by H₂O₂, and then DBT organic sulfide diffuse into mesopores and contacts with catalytic activity center, as a result, electron-rich sulfuratom are oxidized into correspondent sulphone. The final desulfurization wereachieved after the sulphones were extracted out from oil solution using acctonitiles asextracting agents.5. The effect of HPW/SiO₂(E-x%) nanocomposite catalyst on catalytic oxidationin actual petroleum product have also been investigated. By researching on theoxidative desulfurization of straight-run diesel oil with sulfur content of 1800μg/g, itis discovered that the sulfur content of straight-run diesel oil decreases from 1800μg/gto 312μg/g and thedesulfurization rate reaches 82.7% after 4 hours' reaction. Thecatalyst has no evident activity lose after three usages. The influence of hydrocarbonmaterial in diesel fuel on catalytic oxidative desulfurization of petroleum productwere investigated, the author discovers that hydrocarbon of methane series almost hasno effect on oxidation of DBT, while olefin hydrocarbon and aromatic hydrocarbonhave evident effect on DBT oxidation.6. Ag⁺ ion was used to modify the HPW/SiO₂(E-x%) catalyst. Using theЛ-complexing action of ion Ag⁺ to multiring thiofuran, the desulfurization process cancombine adsorption with oxidation. The catalyst can selective oxidize BT and DBTorganic sulfide in diesel fuel, it has achieved excellent desulfurization effect. Afterfour hours of reaction, the sulfur content of straight-run diesel oil decreases from1800μg/g to 228μg/g and the rate of desulfurization rate reach 87.3%. The catalyst hasno evident activity loss after three usages.