THE TEXTURE AND ELECTRON-ACCEPTOR, ACID-BASE, AND CATALYTIC PROPERTIES OF MODIFIED ZSM-5 TYPE ZEOLITES .2. A STUDY OF PYRIDINE ADSORPTION BY IR SPECTROSCOPY AND THERMODESORPTION

IR spectroscopic and TPD measurements of pyridine adsorption were made on a series of ZSM-5 type zeolites prepared by direct synthesis (SiO2/Al2O3 = 28, 34, 44, 56, 81, and 168) and calcined at 500 and 800-degrees-C. It is shown that the total quantity of pyridine sorbed reaches 5 - 7 molecules per unit cell (u.c.) and does not depend on either the SiO2/Al2O3 ratio or the temperature of precalcination. At the same time, the amount of strongly adsorbed pyridine (T(des) = 250 - 600-degrees-C) is independent of the calcination temperature but falls from 2.0 to 0.3 molecules per u.c. as the SiO2/Al2O3 ratio rises. The increase in zeolite calcination temperature results in a growing contribution of Lewis sites. Modification of the outer zeolitic surface by tetraethoxysilane treatment does not noticeably change the total quantity of pyridine sorbed or the number of strongly adsorbed molecules.