Synthesizing Active Alumina By Parallel Flow Coprecipitating Method And Thermal Stability

γ-Al2O3was extensively used as catalyst or catalytic support due to its high surface area,porosity and surface acidity. But in automotive catalytic process, the high temperature phasetransformation of alumina supports occurs result in the loss of alumina activity owing to thesystem temperature can be upgraded to more than1000℃in an instant. Therefore, it hasimportant practical significance to improve the high temperature thermal stability of alumina.This paper mainly studies the synthesis of high performance alumina and the modification ofactivated alumina. Firstly, the reaction conditions of preparation activated alumina includingpH value, reaction temperature, aging temperature and wash way by flow method were studied,the optimal synthetic conditions can be determined through XRD and N2adsorption anddesorption testing, and the product performances were researched combined with TEM, FTIRdetection means; Next, and the variation of improve the thermal stability of activated aluminaby the addition of n-butanol to dry by azeotropic distillation, organic PEG, non-metallicelements silicon and rare earth elements lanthanum and the textural properties of γ-Al2O3wasinvestigated. The results show that:The optimal synthetic conditions include the pH=9, reaction temperature at70℃, agingtemperature at90℃, and the wash agent is hot water and ethyl alcohol. The γ-Al2O3with highsurface area (18.93m2·g-1) and pore volume (1.60cm3·g-1) and little pore diameter(9.60nm) upto500°C is prepared; The specific surface area of the samples after calcined at1100℃is103.61m2·g-1, the crystal form remains as θ-Al2O3; the specific surface area of the samplesafter calcined at1200℃is reduced to69.64m2·g-1with a part of the θ-Al2O3crystal convertedto α-Al2O3. The results of TEM analysis show that γ-Al2O3is the nano fiber structure with sizeabout10nm, and come into nanoparticulate structure with size of about20nm after hightemperature calcination and show a high degree of dispersion and thermal stability.The activated alumina with high specific surface area and pore volume and pore size isobtained by adding different medias to azeotropic distillation drying experiment processes.Experiments showed that the specific surface area of the sample added n-butanol and calcinedat500℃reache to463.80m2·g-1, its pore volume is1.92cm3·g-1and the aperture is11.61nm.After calcination at1100℃, its specific surface area and pore volume still are109.55m2·g-1 and0.81cm3·g-1. As for the activated alumina with the addition of surfactant PEG, which thethermal stability is obviously higher than the activated alumina unadded PEG, PEG caneffectively inhibit γ-Al2O3to α-Al2O3crystal transformation. The optimal added dosage ofPEG is1%and the surface area, pore volume and pore size of the modified sample by theactivation of500℃respectively is530.19m2·g-1,2.16cm3·g-1and13.93nm, after calcinationsat1100℃, the specific surface area and pore volume still keeped115.04m2·g-1and0.91cm3·g-1.The studies of different levels Si-doped activated alumina by impregnation method haveshown that Si doping can significantly improve the thermal stability of activated alumina. Theγ-Al2O3with Si doping amount of8%has the better thermal stabilization after calcination of1100℃, the specific surface area up to161.81m2·g-1, increased by nearly60m2·g-1. When aftercalcination of1200℃, the specific surface area of Si doping amount of6%still be able to keep112.53m2·g-1, and the crystal still is θ-Al2O3. In addition, the γ-Al2O3nanofibers of La dopingresearch shows that the samples with La doping amount of6%has the most prominent thermalstability and the specific surface area maintain at93.07m2·g-1after calcinations at1200℃.Through these studies: The nano-fibrous activated alumina with excellent performancewas acquired by parallel flow coprecipitation under optimum conditions. The thermal stabilityproperties of activated alumina obtained further improved with appropriate doping. So, theγ-Al2O3excellent thermal stability is mainly due to its high thermal stability of nanofiberstructure and elements doped.