Decrease of catalytic activity and solid acidity by ion exchange of Na cation on HZSM-5

Dependence of the cracking activity on solid acidity was studied on HZSM-5 with different concentrations of Na cation. The IR intensity of the hydroxide bond ascribable to Bronsted acid site decreased gradually with increasing degree of exchange of Na cations, and disappeared at an exchange degree of about 80%. Cracking of octane was studied in order to examine the dependence on the number of Bronsted acid site at 773 to 823 K. It was found that the rate of cracking was related to the number of Bronsted acid sites in a more than first-order dependence at 773 K, but was clearly first-order at 823 K. Therefore, the catalytic cracking activity of the HZSM-5 was simply related to the number of Bronsted acid sites only at high temperatures. The rate of cracking was related simply in a first-order with respect to the partial pressure of octane at a temperature as high as 823 K. Solid acidity as measured by temperature programmed desorption of ammonia could not be related simply to the catalytic activity, because the method could not discriminate between Lewis and Bronsted acid sites. (C) 2004 Elsevier B.V. All rights reserved.