Fabrication Of Novel Porous Silica Supported Catalysts And Their Catalytic Performances For Hydrotreating

The development of the world economy has led to the growing demand of transportation fuels,resulting in the more and more serious environmental pollution caused by oil combustion.Countries around the world have legislated increasingly stringent environmental regulations to limit the content of sulfur,nitrogen compounds and aromatics in fuel.On January 1,2019,China has fully implemented the national VI diesel standard,which further reduce the content of polycyclic aromatic hydrocarbons and ensure the sulfur content less than 10?g/g,consequently imposing a severe challenge on the production techniques for clean diesel.The crucial point of producing high-quality clean diesel is developing high-efficiency hydrotreating catalysts.In this thesis,the novel hydrorefining catalysts with high efficiency were designed and fabricated by adjusting support structure and surface properties with the aim of removing high-content macromolecular sulfur compounds and polycyclic aromatic hydrocarbons from inferior diesel oil.Firstly,mesoporous silica-aluminum composite Al-CMS(cubic mesoporous silica)with highly ordered cubic mesostructure and Al-NMS(nano-scaled mesoporous silica)with tunable pore structure and morphology were synthesized.Then a series of NiMo supported catalysts were prepared and used dibenzothiophene(DBT)to be the probe molecule for studying the desulfurization performance and mechanism.Additionally,hierarchically porous composites ZSM-5/NMS(ZN)combined the advantages of mesoporous and microporous molecular sieves were synthesized by introducing ZSM-5 nanocrystals into the NMS mesoporous system,furthermore,amino-functionalized N-ZN materials were obtained by adopting"soft nitriding"method.Then a series of highly dispersed and sulfur-tolerant Pt based catalysts were developed for the hydrogenation of tetrahydronaphthalene,and the hydrogenation activity and sulfur resistance of noble metal catalysts were investigated.The main research contents and results are summarized as follows:(1)Highly ordered mesoporous materials CMS with three-dimensional cubic mesostructure were facilely fabricated by a simple and mild modified-Stober synthesis method,and then CMS were modified by chemical grafting method with Al species,thus the serial mesoporous aluminosilicates Al-CMS-x with various Si/Al molar ratios were obtained and further taken as supports to prepare NiMo supported catalysts.The HDS catalytic activity and reaction mechanism of DBT were studied.Moreover,the structure-activity relationship was deeply discussed based on the characterization results of the serial materials and catalysts.The results showed that Al species mainly existed as framework aluminum in Al-CMS-x materials.The introduction of Al species brought abundant B and L acidic sites for pure silicon-based materials,also could enhance the metal-support interaction and further improve the dispersion of active phases.The support still maintained good porous structure with high surface area(811 m²/g)and pore volume(0.84 cm³/g)when the Si/Al molar ratio reached 10,which would benefit to the formation of well-dispersed and moderately stacked MoS₂ active phases on the support surface.Therefore,the accessible active sites for DBT reactant molecules could increase,and thus enhancing HDS activity of catalysts.Among all of catalysts,NiMo/Al-CMS-10 showed high desulfurization efficiency(93.5%,WHSV=20 h^-1),k_HDS(9.80�10^-4 mol�g^-1�h^-1)and TOF(2.48 h^-1).(2)Ordered mesoporous silica nanoparticles NMS-x with different structures were synthesized in the modified-Stober synthesis system by simply adjusting the amounts of pore expanding agent(n-hexane).With increasing the addition of n-hexane,the mesostructures of materials transformed from helical 2D hexagonal(p6mm)to 3D cubic configuration(Pm3n),the pore sizes increased,and the morphology of NMS-x nanoparticles showed long-rod,short-rod,cubic and spherical shapes respectively.Then,Al-NMS-x supports with different structures were synthesized by Al modification,and the corresponding NiMo catalysts were subsequently prepared through impregnating the active metals of molybdenum and nickel salts.The HDS performance and desulfurization mechanism of DBT were to be studied,the characterization results were deeply analyzed,and the influences of support structure on structure-activity relationship were discussed in detail.As a result,the support structure had significant effects on the morphology and distribution of NiMo S active phases,which in turn influenced on the intrinsic activities of catalysts.In addition,the diffusion properties of reactant molecules were greatly affected by support structures.NiMo/Al-NMS-5 and NiMo/Al-NMS-7 catalysts with three-dimensional mesostructures,larger pore sizes,and well-dispersed II NiMo S active phases,showed higher HDS activities and better mass transfer efficiencies and diffusion performance in the DBT HDS reaction,in which NiMo/Al-NMS-7 catalyst exhibited the highest HDS desulfurization efficiency(95%),TOF(4.8 h^-1),rate constant(1.4�10^-3 mol�g^-1�h^-1),and effective diffusion coefficient(12.8�10⁶ cm²�s^-1).(3)The micro-mesoporous composites ZN-x with different Si O₂/Al₂O₃ molar ratios were synthesized by introducing ZSM-5 microcrystalline units into the synthesis system of NMS with large 3D pore structure.Finally,the advantages of mesoporous materials and microporous molecular sieves were combined.ZN-x composites possessed excellent pore structure properties,showing regular pore structure,high specific surface area,large pore volume and pore size,which would favor to the active metal loading,and the adsorption and diffusion of reactant molecules.Subsequently,Pt supported catalysts were prepared with NMS,ZSM-5 pure material and ZN-x composites as the supports.Tetrahydronaphthalene and DBT were used as probe molecules to evaluate the hydrogenation activity and sulfur resistance of serial catalysts.Combining the characterization results with structure-activity relationship studies,it was shown that,on one hand,the introduction of ZSM-5 nanocrystals enhanced the weak interaction between pure silicon-based materials and metals and further improved the dispersion of Pt particles,which favored the formation of Pt nanoparticles with small sizes;on the other hand,it brought more acidic sites to the ZN-x supports,especially B acid sites,which led to form sulfur-resistant"electron-deficient"adduct with neighboring Pt nanoparticles.The synergistic contribution between mesopores and micropores of ZN-x composites greatly improved the catalytic activity.Among the series of Pt catalysts,Pt/ZN-90 exhibited outstanding hydrogenation activity and sulfur resistance,deriving from its excellent micro-mesoporous structure,suitable metal-support interaction,well-dispersed small Pt particles and enhanced B acid sites.(4)The micro-mesoporous composites ZN were functionalized with low-temperature urea pretreatment via"soft nitriding"method,and a series of amino-functionalized N-ZN support materials with varying degrees of"soft nitriding"were obtained by adjusting the urea/ZN mass ratios.By utilizing the coordination complexation between amino functional groups and Pt components,highly dispersed Pt based catalysts were prepared for investigating the hydrogenation activity of tetrahydronaphthalene and sulfur resistance.The characterization results showed that"soft nitriding"treatment could produce rich nitrogen-containing species on the surface of supports,resulting in a certain degree of pore-channel blockage.With the increase of urea/ZN mass ratio,the pore structure parameters of the materials gradually decreased.By moderatly treating ZN with"soft nitriding"method,the good pore structure properties of N-ZN material were maintained and the amino functional groups were introduced to support surface,further enhancing the affinity of Pt precursors onto supports and therefore improving the dispersion of Pt particles;furthermore,the enough acid sites of N-ZN materials were kept.The highly-dispersed small size Pt particles were formed on Pt/N(0.4)-ZN catalyst,which finally favored to form the"electron-deficient"adducts with sulfur resistance under the function of B acid sites on support surface,as a result,Pt/N(0.4)-ZN showed excellent hydrogenation performance and sulfur resistance.