Strong Brønsted acid site in HZSM-5 created by mild steaming

HZSM-5 was treated with water vapor of less than 0.2 bar (mild steaming). The acidity of the treated HZSM-5 samples was characterized by usual and IRMS–TPD of ammonia, catalytic cracking activity of octane and hexane was measured, and a correlation between the solid acidity and the catalytic activity was studied. The mild steaming changed the Brønsted acidity; the number was decreased but the strength was increased by increasing the partial pressure of water vapor. The cracking activity showed the maximum conversion against the partial pressure of water vapor, as has already been reported. Therefore, the present study confirmed the principal of the mild steaming for the catalytic activity enhancement. The activation energy for hexane cracking was correlated linearly with the strength of Brønsted acidity; the larger the strength of acid site, the smaller the activation energy. First order dependence of the reaction rate of the cracking on the alkane pressure was confirmed irrespective of the treatment conditions. Thus, the catalytic activity of hexane cracking could be simply explained by proceeding of the mono-molecular reaction on the Brønsted acid site. Dislodged Al-containing species most probably enhances the Brønsted acidity, and accordingly the catalytic activity is enhanced, as long as number of the site is kept sufficiently.

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► The acidity of HZSM-5 was changed strongly by mild steaming.
► The changed acidity was found to be Brønsted type.
► Number of the acid site decreased, but the strength enhanced.
► Enhanced acidity was an origin of the enhanced catalytic activity.
► A relationship between acid strength and activation energy was observed for hexane cracking.