Research On Modified Alumina Prepared Through Impregnation

Active alumina has a good advantage in the field of catalyst supports due to its perfect surface area as well as its pore structure and surface acidity. But in some systems when the reaction temperature is very high or with the vapour around, the catalyst was easy to be sintering and had a sharply down of its surface area and pore volume, which made the active site lost and decrease the catalyst activity suddenly. The mechanism need to be discussed in a further way in order to solve the problem.In this paper, P-doped alumina was prepared through impregnation. Common characterizations such as XRD, BET and IR were employed to investigate the effect of phosphate dopant on the crystal phase, surface areas, pore structure and surface acidity of alumina. The results showed that the addition of phosphate could effectively improve the surface areas and thermal stability of alumina. It was 3wt.% P-doped sample that had the best effect because its surface area retained 75.69m2.g-1, the pore volume retained 0.2964 cm3.g-1 and the average pore diameter retained at 15nm after calcination at 1200℃for 3h. We speculated that, phosphor taken as PO43- mode doped in the surface of alumina during the impregnation, and part OH- could be replaced by H2PO4- to delay the dehydration at high temperature. Meanwhile, the addition of phosphate could minish the crystal size to restrain the nucleation process ofα-Al2O3. Phosphor also can steady Al3+ of alumina to prevent the realignment of Al3+, decrease the speed of Al3+ pervasion and restrain the agglomeration of alumina at high temperature.Compared with P-doped alumina, 3wt.% La, Ce, Ba and Ce-Zr doped alumina also were prepared by the same process to investigated the effect on the pore structure and the thermal stability. The result showed that the samples could improve the pore structure and thermal stability of alumina, and La-Al2O3 had the best property, while CZ-Al2O3 and Ba-Al2O3 followed, Ce-Al2O3 worst. But they were all worse than the stability of P-doped alumina due to the difference of modification mechanism.