Synthesis Of Hierarchical Alumina And Its Application In Long Chain Alkane Dehydrogenation Catalyst

The catalysts for heavy oil cracking is easily inactivated due to the diffusion resistance of the heavy oil fractions in the channels of the catalysts,deposition of heavy metal impurities or coke deposition.It is urgent to develop new ?-Al₂O₃ materials with large pore volume and high surface area,which is widely used as the catalyst carrier in the petroleum and chemical industries.In this thesis,a series of hierarchical micro-meso-maro-?-Al₂O₃ with large pore volume,large pores and high specific surface area were prepared,and the adsorption and diffusion properties and its application as catalyst in catalytic dehydrogenation of long chain alkanes were systematically investigated.A series of hierarchical micro-meso-maro-?-Al₂O₃ with average pore size 22.6 nm,pore volume 1.59 cm³/g and surface area 301.1 m²/g were prepared by chemical precipitation method,and the conditions for synthesizing were optimized by the orthogonal design method.Furthermore,in order to increase the ration of macro-pore?special for the pore size larger than 300 nm?,the reconstructing the precursor of alumina method was used to prepare hierarchical micro-meso-maro-?-Al₂O₃ with concentrated mesoporous distribution and wide distribution of large pore.In this method,aluminum ammonium carbonate is used as the reaction intermediate,and thus induced to the increase of the mount of AACH in the precursor of alumina and decrease of amorphous AlOOH.With a significant increase in macro-pore,the average pore size,pore volume and surface area of thus prepared ?-Al₂O₃ can get to 33.9 nm,2.17 cm³/g,284.6 m²/g respectively.The hierarchical ?-Al₂O₃ was also has been prepared by reconstructing the structure of alumina in order to resolve the problem that the macro-pore size distribution is too broad.In this method,aluminum ammonium carbonate is also used as the reaction intermediate.A great amount of AACH precursor was produced,after amorphous alumina re-dissolution and re-precipitation process in ammonium bicarbonate due to the memory effect,and thus prepared ?-Al₂O₃ with the average pore size 22.4 nm,pore volume 1.95 cm³/g and surface area 322.5 m²/g,has several kinds of pore size distribution centered at 6.9 nm,80.6nm,3152 nm respectively and a small pore size distribution centered at 22 ?m.Above two kinds of reconstruction methods also have undergone a process of a dissolution-precipitation process,and thus changed the way of the ?-Al₂O₃ particle accumulation.In the thesis,a series of micro-nano-scale hierarchical micro-meso-maro-?-Al₂O₃ with variety of morphology were prepared by template method,hydrothermal method and in-situ synthesis method.Firstly,monodisperse submicrometer polystyrene?PS?microspheres were prepared by soap free emulsion polymerization method and then the hierarchical ?-Al₂O₃ was prepared by three methods: 1)Monodisperse polystyrene?PS?emulsion or PS colloidal crystal as template,hierarchical micro-meso-maro-?-Al₂O₃ with regular pore structure was prepared;2)The hollow spherical ?-Al₂O₃ was in-situ synthesized on the PS microsphere surface by precipitation method;3)The flower-shaped hollow hierarchical micro-meso-maro-?-Al₂O₃ was prepared by hydrothermal method.TiO₂ was often used to modify the supported metal and enhance the catalytic dehydrogenation performance of the catalysts due to its properties such as reducible surface and possible electron transfer by spontaneous alignment of the Fermi levels.Titanium-aluminum composite ?-Al₂O₃ carrier was prepared by impregnation and co-precipitation method.The loading of TiO₂ was 5 %,10 %,15 % respectively.The ?-Al₂O₃ was characterized by XRD,N2 adsorption,SEM,mercury porosimetry,FT-IR,Thermogravimetry,CO pulse adsorption,H₂-TPR,NH₃-TPD,particle size analyzer and the results show that TiO₂ is well dispersed on the surface of ?-Al₂O₃ and the loadings of TiO₂ has almost no influence on the pore properties of ?-Al₂O₃.The surface acid strength and acid distribution on titanium-aluminum composite ?-Al₂O₃ almost has no any change,but for different pore structure carrier,the change trend of total acid amount is different.In summary,after loadings of TiO₂ on the ?-Al₂O₃,the surface acid on titanium-aluminum composite ?-Al₂O₃ almost has no any change.Above ?-Al₂O₃ as carrier,a series of long chain alkane dehydrogenation catalysts were prepared,and the catalytic dehydrogenation performances of above catalysts were systematically investigated using fixed bed technology.The results show that the pore size distribution has significant influence on the dehydrogenation performance of the catalysts.The catalysts with C4 and C8 posing hierarchical micro-meso-maro structure as carrier show better dehydrogenation performance.Furthermore,the connected maro-pore size distribution is very beneficial to the stability of the catalyst,special for micrometer scale macro-pore,which can contribute to the migration of the coke in metal active sites.The titanium-aluminum composite ?-Al₂O₃ as carrier,the interaction between Pt-Sn-TiO₂ and Pt-Sn-Al₂O₃ is very strong when the loadings of TiO₂ is less than 5%,and thus producing “double sandwich” structure.The interaction between Pt-Sn-TiO₂ and Pt-Sn-Al₂O₃ is beneficial to the dispersion of Pt on the surface of the catalyst,and thus is helpful to the stability of the catalyst.Meanwhile,the adding TiO₂ has changed the electronic state of Sn,and improved the dehydrogenation performance of the catalysts due to inhabiting the reduction of Sn oxides.When the loadings of TiO₂ increase to 15% from 10%,the interactions between Pt-Sn andTiO₂ are strengthened and the interactions between Pt-Sn and Al₂O₃ are weakened due to SMSI?Strong-Metal-Support-Interaction?effect,and thus the stable Pt-Sn-TiO₂-Al₂O₃ structure is destroyed companied with hydrogen spillover,which results in the activity of catalyst is decreased.The catalyst modified by 5 % TiO₂ shows excellent dehydrogenation performance.Among the titanium-aluminum composite catalyst,Cat-C3-10PS-5Ti has excellent activity?15.06 %?,but with lower stability?26.78 %?.Both Cat-C4-5Ti and cat-C3-20PS-5Ti have excellent activity?14.97 % and 14.01 % respectively?and stability?18.81 % and 19.53 % respectively?,and the stability is slightly lower than that of industrial catalyst?18.20 %?,but the activity is super to that of industrial catalyst?13.57 %?.