Acidity and catalytic activity of mesoporous ZSM-5 in comparison with zeolite ZSM-5, Al-MCM-41 and silica-alumina

Abstract

Acidity of mesoporous HZSM-5 prepared using amphiphilic organosilane template molecules was measured. Bronsted acid sites were observed in the prepared sample, and the number and the strength of Bronsted acid sites were determined quantitatively by a method of infrared-mass spectroscopy/temperature-programmed desorption (IRMS-TPD) of ammonia. Delta H for ammonia adsorption as an index of the strength was ca. 150 U mol(-1) that was almost the same as on usual HZSM-5, but the number was smaller than that of HZSM-5. From the measured acidity, it was concluded that the mesoporous materials contained a smaller concentration of Bronsted acid site notable on the structure of HZSM-5. Measurements of turnover frequency (TOF) in the catalytic cracking of octane supported the conclusion. Density functional calculations showed that the defect sites Al-OH and Si-OH attached to the Bronsted acid site changed the strength of the acid sites to show some possible structures of the weak and strong Bronsted acid sites included in the mesoporous HZSM-5. Acidities of Al-MCM-41 and silica-alumina were also measured, and the difference in the solid acidities of these materials was discussed. (C) 2008 Elsevier B.V All rights reserved.