| In this paper, through modifying ZSM-5 zeolites with silica by chemical liquid deposition (CLD) of dirnethyl silicone oil, the selective toluene disproportionation catalysts have been prepared. The reaction performances of catalysts were evaluated by a fixed-bed micro-reactor operated under medium pressure. Several factors such as the included cation of zeolites, the molar ratio of silica to alumina, the calcinating atmosphere in the process of modification, the viscosity of silicone oils, the deposition amount of silica, the properties of non-zeolitic powders, which affect the effect of modification have been studied. The secondary reactions have been tested by feeding pure para-xylene as reactant. By means of JR. nitrogen adsorption, 27A1-NMR, TEM, 3-TPD, DTA and the explorer molecular reactions of metaxylene and 1 ,3,5-triisopropylbenzene, the properties of catalysts and modified catalysts were characterized. According to these studies, the mechanistic models of modification affects zeolite acidity and pore structure, the mechanistic model of para-selectivity caused by modification and the model of the secondary reactions have been approached. It is shown that under the catalysis of hydroxides on the external surface of zeolite, the silica membrane can be obtained by the thermolysis of silicone oil, and the homogenizing deposition of silica membrane is induced by the thermal oxidation of silicone oil. The narrowing of the pore-opening size and the inactivation of the external surface acidity caused by silica account for the high para-selectivity. The weak thermal oxidation stability of silicone oil, the acid binder, and the high silica-to-alumina ration, otherwise, have negative effects on the increasing of para-selectivity. Several secondary reactions take place on the catalysts, mainly including xylene dealkylation, transalkylation, hydrocracking reactions, and the acid property and the level of modification influence the secondary reactions. |
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