| Popis: |
The isomerization of meta-xylene under normal pressure was studied in the temperature range 570–770 K on three catalyst series, viz. (i) gallosilicates with a constant Si/Ga ratio of 50, but different synthesis times ranging from 5 to 96 h, (ii) gallosilicates with a higher (100) and lower (25) Si/Ga ratio after 48 h and 96 h crystallization time, and (iii) a series that included a ZSM-5 zeolite, a gallosilicate, and a ferrisilicate, all with comparable concentrations of Bronsted acid sites. The characterization of the catalysts is based on XRD, electron microscopy (TEM, SEM), ammonia thermodesorption, thermoanalysis and n-hexane adsorption. Synthesis products that were characterized by X-ray diffraction to have 100% crystallinity, acquire further acidity when the synthesis time is prolonged. This effect is due to an additional insertion of gallium in framework positions from a reservoir of nonframework gallium species. — The isomerization on gallosilicates proceeds with negligible disproportionation to toluene. The activity is intermediate, compared to the ZSM-5 zeolite and to the ferrisilicate. The shape selectivity of the gallosilicates is inversely related to the crystallinity. — The kinetics of the isomerization can be described by a parallel two step model, in good agreement with experimental results. The significance of internal mass transport effects is discussed. |
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