Abstract
A reaction mechanism for the alumination of highly siliceous HZSM-5-type zeolites with AlCl 3 was proposed. The amount of aluminum introduced into the framework at 923 K leveled off at a reaction time of I h and a partial pressure of AlCl 3 of 11 kPa, and was independent of the number of terminal silanol groups and of the amount of silicon released from the zeolitic framework. From these results, it is concluded that the alumination proceeds not through the substitution of silicon by aluminum, but through the insertion of aluminum into the defect sites. The acidic properties of the aluminated HZSM-5 were examined by pyridine-IR spectroscopy. Strong acid sites including both Brønsted and Lewis acid sites were generated by the alumination. Aluminated HZSM-5 zeolites exhibit a higher activity per framework aluminum for octane cracking than do HZSM-5 zeolites.
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