Study On The Synthesis Of Hierarchically Structured Alumina And Their Applications In The Hydrotreating Catalysts

Along with the increasing exploration of the crude oil,the amount of light oils decreases,which meanwhile leads to increasing proportion of residual oil.The composition of residual oil mainly consists of a large amount of residual carbons,sulfur,heavy metals,nitrogen and other impurity elements,and there are also undesirable components such as colloids and asphaltenes.Therefore,the design of the alumina with high permeability characteristics such as hierarchical porous structure with large pore volume is highly desired.This thesis mainly focuses on the research of hydrothermal fabrication of a series of alumina carriers with hierarchical structure using cost-effective inorganic aluminum source and urea as raw materials;and the formation mechanisms of the as-obtained hierarchically structured alumina with different morphologies were explored in detail.On this basis,a series of hydrotreating catalysts were prepared with the hierarchically structured aluminas as carrier and Ni and Mo active components loaded;the catalysts were studied by a series of characterization techniques and the relationship between the hierarchical structures of the catalysts and their catalytic properties in hydrotreating reactions were also investigated.Firstly,nest-like hollow ?-AIOOH microspheres constructed from numerous hierarchically organized nanowires were synthesized via a template-free hydrothermal approach and were well characterized.The as-prepared alumina sample had two pore size distribution peaks with average macropores of 900 nm,mesopores of 20 nm anda pore volume of 0.93 cm3·g-1.The mesopores were formed by the winding nanowires,while the macropores originate from the hollow nature of the microspheres.Due to their structural merits,such nest-like ?-Al2O3 microspheres could serve as excellent catalytic support,and a MoNi/?-Al2O3 supported catalyst was prepared,which was applied for hydrodemetallization(HDM)catalysis.Compared with the catalyst prepared with the commercial ?-Al2O3,our catalyst exhibits superior catalytic performance and longer life due to enhanced diffusion of the reactants and dispersion ratio of active metals.Furthermore,the formulation for synthesis of the above macro-mesoporous nest-like alumina was further investigated by adjusting the dosage of the citric-acid applied,which leads to the formation of a series of hollow alumina microspheres.With different dosages of citric acid applied,alumina microspheres were assembled with different specific surface areas,pore volumes and acidity owing to the chelation between citric acid and aluminum ions which inhibit the polymerization of the primary particles of alumina.After the loading of the MoNi active components,a series of hydrodesulfurization(HDS)catalysts were obtained and characterized systematically by various relevant techniques;and their catalytic activity and selectivity towards HDS of dibenzothiophene(DBT)were evaluated and compared.It is revealed that,the macro-mesoporous MoNi/Al2O3-xCA(x denotes the gram of citric acid(CA)added in each synthesis ranging from 0 to 0.6 g)catalyst with high mass transfer efficiency shows excellent HDS performance compared to the single mesoporous MoNi/cAl2O3.The catalytic efficiency of the catalyst highly depends on the factors including the specific surface area and the acidity,the sulfidity and the dispersion of the active metal components.Moreover,in another closely related issue,for the preparation of industry alumina-based catalysts,pore-directing agents are often introduced together with the precursor materials during the molding process to induce the formation of macropores.However,for such a practice the pore-directing agents need to be removed via calcination,and the resulting macro-mesoporous alumina carriers usually suffer from a small bulk density,a weak mechanical strength and fragile pores which tend to collapse.Towards the pursuit of industry catalysts,the aforementioned mesoporous alumina microspheres prepared via hydrothermal methods(each of which is an assembly of numerous highly crystallized alumina nanorods)were further assembled in one step via pore-directing-agent-free molding process,such that affluent inter-micro sphere macropors would be induced in the resulting assemblies.Owing to the morphological merits and the remarkable mechanical stability of the as-formed alumina microspheres,the macropores formed from the stable packing of the alumina microspheres have average sizes as large as 265 nm and are well connected with each other;meanwhile,the intra-microsphere mesopores with an average size of 12.2 nm are well maintained after the molding process.With such hierarchical porosity and large specific surface area,the as-prepared alumina support enables better dispersion of the active MoNi components.Accordingly,the macro-mesoporous MoNi/Al2O3 catalyst exhibited significantly higher catalytic activities and longer service life both for HDM of nickel(?)5,10,15,20-tetraphenylporphine(Ni-TPP)and HDS of DBT than those controls with a single type of pores.With ease of preparation,large surface area,and remarkable activity and stability,such macro-mesoporous alumina support provides a promising alternative for hydrotreating of heavy raw oils in industry.Last but not least,the formation mechanism of hollow alumina microspheres was discussed.Based on with the above research results,it is demonstrated that the physicochemical properties of the amorphous microspheres formed in the initial stage of the hydrothermal reaction plays an important role in determining whether the eventual hollow alumina microspheres can be successfully formed or not.And the hydrothermal formation of hollow alumina depends on the factors including the temperature,the reactant concentration and the pH value.Moreover,to exlain the formation of the hollow interior of the alumina microspheres,the ripening mechanism based on the difference in particle surface free energy was proposed in comparison to the classical Ostwald ripening mechanism which is based on the differentiated particle diameter.Furthermore,a mathematical model was also developed to describe the influencing factors on the size of amorphous alumina microspheres,which is expected to provide theoretical guidance for controllable synthesis of hollow alumina microspheres.